Preheat the oven to 350 degrees F. Butter the bottom and sides of a 9-inch springform pan. Cover the bottom of the pan with a round of parchment, then butter the parchment and dust the parchment and sides of the pan with cocoa powder. Knock out the excess and set aside.
Combine the chocolate and butter in the top of a double boiler set over simmering water. Melt them together, stirring occasionally, until smooth. Remove from the heat and reserve.
Briefly whisk together the sugar and eggs in a bowl. Pour the melted chocolate into the egg mixture and stir to combine.
Pour the batter into the prepared pan and wrap the bottom tightly with aluminium foil. Place the pan inside a bigger baking pan, then add hot water to the larger pan, making sure it comes at least halfway up the side of the springform pan.
Bake the cake in its water bath until set in the center, about 1 hour and 15 minutes. Let cool to room temperature, then refrigerate for 1 hour.
Run a knife or offset spatula around the pan edge to loosen the cake from the pan. Sift cocoa powder over the top of the cake and serve.
This cake keeps well, wrapped and refrigerated, for up to 1 week.