Pre-heat the oven to 160°C (140 fan)/ 325 degrees F or Gas Mark 3.
Grease your tray and line with baking paper. I used a 15" x 10.5 and 1.5" deep square tin. Set aside.
For a soft and spongy cake make sure that you have left the butter in room temperature. Cream the butter and sugar together until smooth. Add the eggs one by one and mix well. Once all the eggs have been added, add in the flour using 2 tablespoons at a time to make sure the mixture is smooth. Add in the milk and vanilla essence and mix well.
Pour the sponge cake mixture into the tray and bake for around 25-30 minutes. Keep and eye on the cake until it turns golden brown. You can see if the cake is done by sticking a toothpick or a knife into the middle of the cake. If it comes out clean then your sponge cake is done. Leave the cake to cool in the tin for about 15 minutes before turning it out onto a wire rack.
Once the cake has fully cooled down, make the icing by adding the milk into the icing sugar. Add half of the milk first to test the consistency. I like my icing to be quite thick, once the icing is done pour it in to the middle of the cake and spread carefully with a spatula. Let the icing run along the sides of the cake for effect. Then add the sprinkles!