1. Preheat the oven to 180C/160C Fan/Gas 4. Grease a 4cm-deep rectangular tin, 28cm x 19cm, and line with baking paper.
2. Melt the butter in a small pan. Remove from the heat and stir in the cocoa. Set aside for 10 minutes.
3. Whisk the eggs, sugar and vanilla together in a large bowl until pale and foamy.
4. Stir in the cocoa mixture, flour and chillies. Turn into the tin and bake for 35 minutes or until the top is firm but the middle is soft.
5. Cool for 10 minutes, then turn out on a wire rack. When cold, cut into squares and dust with icing sugar.