Preheat the oven to 350°F (180°C). Butter and line a 12-inch (30cm) round and 3-inch (7.5cm) deep cake pan with parchment paper.
Melt the chocolate with the butter in a heatproof bowl set over a pot of barely simmering water (the water should not touch the base of the bowl). Remove the bowl from the pot and cool a little, then whisk in the egg yolks. Add the sugar and cocoa powder and 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. 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, then weight the plate. Leave to cool for 30 minutes before turning out. Cool completely before serving.