Preheat the oven to 180 degrees Celsius or gas mark 4.
Line a 20cm x 20cm square tin with baking paper.
Cream the butter and sugar together in a mixing bowl. Add the eggs, vanilla extract, baking powder and the flour gradually, folding as you go.
Pour the mixture into the tin and bake for 35-40 minutes or until a cake tester comes out clean.
Leave the cake to cool before placing on a cooling rack.
Mix together the icing sugar and water and pour onto the cake, topping generously with sprinkles. Leave to set.
Carefully trim off the edges to make a neat square and slice into 9 even pieces.