1. Whip the egg whites until stiff.
2. In a glass bowl, melt the butter and the chocolate for 2 minutes in the microwave, at the highest setting. Stir, then, still stirring, add the sugar, the flour, the cornflour, the baking powder and the egg yolks. Delicately add the stiff egg whites, folding them in with the help of a wooden spatula.
3. Grease the inside of the deep lid, pour in the mixture and cook in the microwave at the highest setting for 8 minutes.
4. Leave to cool for 10 minutes and remove from the mould.