2. Combine chocolate and butter in a heatproof bowl. Place over a pan of gently simmering water, stirring until melted and smooth. Remove from heat and stir in cocoa.
3. Using an electric mixer, beat eggwhites with cream of tartar until soft peaks form. Add sugar, 1 tablespoon at a time, beating constantly until thick and glossy.
4. Stir a large spoonful of eggwhite into chocolate mixture, then fold this into remaining eggwhite. Spoon mixture into muffin pans and bake for 7 minutes, until tops are cracked and centres are still soft and gooey. Remove from oven, cool in pan for 1 minute and turn out onto serving plates. Dust with icing sugar and serve with a dollop of ice-cream or thick cream.