Melt chocolate and butter together, then cool slightly
Whisk egg whites with one-third of the sugar to soft peaks. Then, whisk yolks with remaining sugar until thick and pale
Stir melted chocolate and butter into yolks, then fold in egg whites and cocoa powder
Pour half the batter into a lined 20cm tin. Bake at 170°C fan for 45 minutes, until set. Cool slightly and press edges flat
Lower oven to 160°C fan. Pour in remaining batter and bake 25-30 minutes, until edges are set and middle is soft
Cool before slicing
