This recipe makes an extra light and fluffy version of this simple classic - topped with mini eggs, it's a showstopping Easter cake or with simple sprinkles, it's a traditional birthday cake.
In another bowl mix together the sugar, vanilla, melted butter, buttermilk, oil, and all six egg yolks. Set aside.
Using a food processor or mixer, whip the egg whites (from the 3 separated eggs) until stiff peaks are formed, set aside.
Melt the butter and chocolate chips in a small pan on over a low heat stirring gently until melted.