This vanilla sponge tray bake has been a pocket of childhood memory for me ever since school dinners dished up rectangles topped in glossy icing and a generous shower of hundred-and-thousands. I first recreated this one rainy afternoon when I wanted to make something that felt like school assembly singing and playground chatter: simple, unfussy, and reliably cheerful. The texture is light and springy, the crumb tender without being fragile, and the icing adds a perfectly sweet, slightly crackly finish that kids love and adults find pleasingly nostalgic. I call it a tray bake because it's baked in a shallow tin and sliced into rectangles — an ideal format for parties, bake sales, or a generous afternoon tea. I discovered that keeping the margarine at room temperature and beating until pale makes the crumb silkier, and a splash of milk at the end takes away any dryness. The recipe is forgiving: measure in cups, and it still behaves the same way. It’s the sort of baking that doesn’t demand technical precision but benefits from little habits — like sifting the powdered sugar for the icing so it pours smoothly and using a skewer to confirm doneness. This version yields 16 slices, and in my house a single sheet will vanish within a day whenever I say the word "sprinkles." The bright finish brings smiles, and the easy method keeps me coming back when I want to make something both quick and heartwarming.

Ingredients

• Self-raising flour: 2 cups plus 2 tablespoons (about 2.1 cups). Use a reliable brand of all-purpose self-raising flour or make your own by combining plain flour with baking powder; sift before measuring to keep the cake light and even.
• Caster sugar (superfine): 1 1/4 cups. If you don’t have caster sugar, use granulated sugar and pulse briefly in a blender to break the crystals — it helps the cake become softer.
• Margarine at room temperature: 1 cup plus 2 tablespoons (about 8.8 ounces). Bring the margarine out of the fridge an hour ahead so it creams easily; soft-not-melted is the right texture for aeration.
• Eggs: 4 large eggs. Use eggs at room temperature so they incorporate smoothly and help the batter emulsify.
• Vanilla extract: 1 teaspoon. A good-quality vanilla extract adds warmth; substitute with vanilla paste for flecks of real vanilla if you prefer.
• Milk (optional): 1 tablespoon. A splash of milk improves the crumb and keeps the tray bake moist — especially helpful if your margarine is lower in fat.
• Icing sugar (powdered sugar): 1 2/3 cups (about 200 grams). Sift first to avoid lumps so the icing pours smoothly.
• Warm water: 7 tablespoons, added gradually to reach a glossy, pourable consistency.
• Hundreds and thousands (sprinkles): As needed for decorating. Choose non-melting, sugar-based sprinkles so they keep their colour when the icing sets.

Instructions

Preheat and prepare: Preheat the oven to 350°F (180°C) or 320°F (160°C fan)/gas mark 4. Line a 9x13-inch (23x33 cm) baking pan with parchment paper, letting the paper hang over the sides for easy removal. Proper lining prevents sticking and gives straight edges when you slice the tray bake. Cream margarine and sugar: In a large mixing bowl, beat the room-temperature margarine with the caster sugar until pale and fluffy, about 3–5 minutes with an electric mixer on medium speed. The goal is to incorporate air — look for a lighter colour and slightly increased volume. If you mix by hand, beat vigorously for around 6–8 minutes. Incorporate the eggs: Add the eggs one at a time, beating each in fully before adding the next. This prevents the batter from splitting. If the mixture looks curdled at any point, add a tablespoon of sifted flour or a splash of milk to bring it back together. Fold in the flour and flavouring: Sift the self-raising flour and fold it in gently in two or three additions using a spatula, using a lifting motion to preserve air. Stir in the vanilla and the optional tablespoon of milk at the end; the batter should be smooth and drop off the spatula with a slow ribbon. Bake: Pour the batter into the prepared pan and level the top with an offset spatula. Bake for 25–30 minutes in the centre of the oven. Test with a skewer in the middle — it should come out clean or with a few dry crumbs attached. If needed, give it another 3–5 minutes, but avoid overbaking to keep the sponge tender. Prepare the icing: While the sponge cools completely in the pan, sift the icing sugar into a bowl and whisk in 7 tablespoons of warm water a little at a time until you have a glossy, pourable icing. Aim for the consistency of heavy cream so it spreads but doesn’t run off the sides. Ice and decorate: Once the tray bake is completely cool, pour the icing over the top and spread gently to the edges. Immediately scatter hundreds and thousands over the surface while the icing is still wet. Leave to set for at least 30 minutes before slicing into 16 squares with a sharp knife.

Storage Tips

Store at room temperature in a single layer inside an airtight container to protect the icing finish — refrigerating will make the sponge firm and dry. If you must refrigerate in a warm climate, wrap tightly and give it time to return to room temperature before serving. For longer storage, slice into portions, wrap each piece tightly in cling-film and place in a freezer-safe container; they’ll keep well for up to 3 months. When reheating, allow frozen slices to thaw at room temperature and serve as-is or warm briefly in a low oven for five minutes to refresh the texture.

Ingredient Substitutions

You can use unsalted butter instead of margarine for a richer flavour — reduce added salt in other recipes accordingly. If you don’t have self-raising flour, combine 2 cups plus 2 tablespoons of plain flour with 3 teaspoons of baking powder. For a dairy-free version, ensure the margarine is labelled dairy-free and the sprinkles contain no milk derivatives. Swap vanilla for almond extract for a different flavour profile, but use sparingly as almond is stronger.

Serving Suggestions

Serve slices with a hot pot of tea or a tall glass of milk for classic comfort. For a more grown-up tea service, dust the edges with finely grated lemon zest before slicing or serve with a side of slightly tart raspberry jam to cut the sweetness. For parties, arrange the slices on a platter with fresh berries and mint for colour contrast and freshness.

Cultural Background

The tray bake is a cornerstone of British home baking and school-dinner culture — humble, economical and designed to feed many. This format evolved from wartime and post-war home baking when simple ingredients needed to make large quantities without fuss. The sugary icing and sprinkles are a later, celebratory addition, popular with school caterers and home bakers who wanted to give children a treat in an affordable way.

Seasonal Adaptations

In spring and summer, fold grated lemon or orange zest into the batter and swap the white icing for a thin lemon glaze. In autumn, stir a pinch of cinnamon into the batter and use caramel-coloured sprinkles. For winter holidays, use red and green decorative sugars and add 1/4 teaspoon of mixed spice to the mix — these small swaps keep the base method intact while nodding to the season.

Meal Prep Tips

This tray bake is ideal for meal prep: bake on a Sunday, slice into portions and store for snacks during the week. For lunchboxes, wrap slices individually to keep them fresh. If you want to layer in fillings later, spread a thin layer of jam or buttercream between slices before freezing so you can thaw a ready-to-eat treat.

There’s something quietly wonderful about a simple sheet of sponge iced and sprinkled — it proves that joy doesn’t always come from complexity, but from technique, memory, and sharing. I hope this version becomes a reliable favourite in your kitchen and brings as many smiles and small celebrations as it has in mine.