Preheat your oven to 180°C (160°C fan) or Gas Mark 4.
Grease and line a 9x13-inch traybake tin with baking paper.
Cream together the softened butter and caster sugar until pale and fluffy.
200 g Unsalted butter, 200 g Caster sugar
Add the eggs one at a time, mixing well after each addition.
4 large Eggs
Stir in the vanilla extract.
1 tsp Vanilla extract
Gently fold in the self-raising flour using a spatula or spoon.
200 g Self-raising flour
Add milk to loosen the batter slightly for a soft pourable consistency.
2 tbsp Whole milk
Pour the batter into your lined tin and level the top.
Bake for 25–30 minutes until golden and springy to the touch.
Allow to cool in the tin for 10 minutes before transferring to a wire rack.
Once completely cool, mix icing sugar with water/milk until smooth.
250 g Icing sugar, 2–3 tbsp Water or milk
Spread over the sponge evenly, then add sprinkles immediately.
2–3 tbsp Rainbow sprinkles
Let the icing set before slicing into squares.