Preheat the oven to 180'C / Fan 160'C / Gas Mark 4. Grease and line a 9-inch x 12-inch rectangular tin with baking paper.

Mix the gluten free flour, baking powder and xanthan gum together in a bowl and set to one side.

Add the caster sugar and Stork to a large mixing bowl and beat with an electric whisk for a minute until creamed together.

Add the eggs, vanilla extract and milk to the mixture and then sift in the flour mix. Beat again with the electric whisk until the mixture has combined to form a smooth batter.

Pour the mixture into the cake tin and smooth into an even layer. Bake in the oven for 35-40 minutes until golden on top and springy to the touch. A skewer inserted in the middle should come out clean.

Remove the cake from the oven and place on a cooling rack. Leave to stand until the tin is cool enough to remove, then carefully remove the cake from the tin and leave the sponge to cool completely.

Add the icing sugar to a bowl and gradually add the water, a little at-a-time, until you have a thick, runny paste. Pour over the cake and spread out to the edges in a thick layer. Liberally cover with sprinkles and leave to set before cutting into squares.