Preheat the oven to 180˚C (350˚F). Lightly grease and line the base and side of a 23 centimeter (9 inch) spring-form cake tin with baking paper.
In a medium saucepan, melt the butter over low heat then take off the heat, add the chocolate and stir until melted.
In a small saucepan, mix together the sugar and 60 milliliters (2 fluid ounces, 1/4 cup) water. Bring to a boil over medium-high heat.
Pour the hot syrup over the melted butter and chocolate and stir until combined.
Stir in the egg yolks, then pour into a medium bowl and set aside.
Use an electric mixer with a whisk attachment to whisk the egg whites until stiff, but not dry, peaks form.
Using a rubber spatula, gently fold the beaten egg whites into the chocolate mixture until combined, making sure there are no streaks of egg white.
Pour half of the mixture into the prepared tin, gently spread level.
Bake for 30 minutes, or until a skewer inserted into the middle comes out almost clean. Remove from the oven and let cool completely.
Flatten the top of the cake, pour over the remaining batter then return to the oven and bake for another 25-30 minutes.
Leave to cool completely before removing from the tin.
Dust with cocoa powder and serve.