Preheat the oven to 180°C and grease and line a 20cm square springform baking tin.
In a large mixing bowl whisk butter and caster sugar until creamy. Add the eggs 1 at a time and continue to whisk until fully combined. Add the dairy free milk and vanilla extract and whisk again.
Fold in the self-raising flour, xanthan gum and baking powder. Mix well until combined and then spoon the mixture into the lined baking tin.
Bake in the oven for 40 minutes, or until golden brown and a skewer comes out clean (cover the top with foil if it starts to brown too quickly). Leave to cool in the tin for 15 minutes before turning out onto a wire rack to cool completely.
Make the icing by combining icing sugar and boiling water until you have a runny (but not too runny!) consistency. Spread on top of the cake and top with sprinkles. Leave to set and serve with dairy free custard.