Chocolate & Vanilla Marble Cupcakes

I saw these in my Hummingbird Bakery recipe book and thought I have to have a go, as I've never made a marble type cake before. 

Hummingbird Bakery recipes are usually very American, therefore use plain flour with baking powder instead of self raising flour. They also use milk as well as butter. I personally don't like it very much as I find they make the cake too heavy & dense - I usually prefer using a lighter good ol' victoria sponge recipe - 6oz flour, 6oz sugar, 6oz butter, 3 eggs.

They came out OK apart from putting too much of the egg/milk mixture in the vanilla cake batter resulting in them not marbling very effectively - boooo! It's also important that the chocolate and vanilla icings are exactly the same consistency else it's a nightmare to get them to swirl nicely when icing the cupcakes.