Preheat the oven to 170°C and grease and line a cake tin slightly smaller than your pie dish.
Melt the butter in the microwave and set aside to cool slightly.
Using an electric mixer, beat the egg whites. Once they begin to foam, add the sugar a little at a time. Continue to beat until stiff peaks form. Add the egg yolks and vanilla and beat on high speed for 5 seconds.
Sift in the flour, baking powder, and cocoa powder into the egg mixture, then use a spatula to gently fold them in until just incorporated (it’s important not to over mix).
Mix a couple of tablespoons of the cake mix with the melted butter until homogenous then fold back into the cake mix.
Pour into prepared cake tin and bake for 20 minutes.
Allow to cool completely, then unmould it and slice it horizontally so you have two cake layers.