Pre-heat the oven to 350°F.
Butter and line a 12-inch round and 3-inch deep cake pan with parchment paper.
In a heatproof bowl, set over a pot of barely simmering water (the water should not touch the base of the bowl), melt the chocolate with the butter.
Remove the bowl from the pot and cool a little; whisk in the egg yolks.
Add the sugar and cocoa powder; mix well.
In a separate large bowl, whisk the egg whites until they form soft peaks.
Fold into the chocolate mixture, a third at a time.
Pour the mixture into the prepared cake pan.
Place in pre-heated oven and bake for about 30 minutes or until the cake has risen like a soufflé and is slightly set.
Remove from the oven; place a piece of parchment paper on top of the cake; then place a plate on top of that—the plate should fit exactly inside the pan—and press down firmly; weight the plate with a heavy object.
Leave to cool for 30 minutes before turning out.
Cool completely before serving.
