Pre-heat the oven to 180 C / Gas 4. Lightly grease a 23cm (9in) round cake tin.
Mix the granulated sugar, caster sugar and melted butter together in a small bowl.
Mix flour and baking powder together in a large bowl. Make a hole in the middle. Add the melted butter mixture and mix gently.
Whisk egg whites until almost stiff. Gradually add the yolks; continue whisking to reach a deep orange colour. Fold this mixture into the cake mixture, adding as much milk as needed to create a creamy, slightly running mixture. Keep the rest of the milk for the icing.
Pour mixture into the cake tin. Bake in oven for 40-45 minutes until golden brown and a skewer inserted into the middle of the cake comes out clean.
For Icing: In a clean bowl, mix icing sugar, vanilla extract, a few tablespoons milk and softened butter. Whisk together to form a thick, creamy icing. Spread evenly on top of the cake when it has cooled.