CLEMENTINE TRES LECHES CAKE

unsalted butter, for greasing • 4 large eggs, separated • 220g golden caster sugar • 150ml whole milk • 120g plain flour • 1 tsp baking powder • 1 tsp vanilla extract • 8 clementines • 150ml sweetened condensed milk • 170g can evaporated milk • 300ml double cream, lightly whipped