Beat together 200g sugar, cocoa powder, hot water and 1 teaspoon vanilla extract with an electric mixer on medium speed until the sugar dissolves, about 3 minutes. Pour this batter into the pastry cases.
Sieve together the flour and baking powder; set aside. Cream together 200g sugar and butter. Beat in the vanilla, then the eggs, one at a time. Slowly beat in half of the milk, then half of the flour until moistened. Mix in the remaining milk then flour. Carefully spoon this batter on top of the chocolate batter, making sure that the entire top of the cake is covered with no chocolate visible.
Bake in preheated oven until a skewer inserted into the centre of the cakes comes out clean, about 30 minutes. Remove from the oven and allow to cool for at least 1 hour before serving.