Preheat the oven to 180°C (360ºF). Cut the butter and break the chocolate in small pieces. Melt them together in a bain-marie.
Mix the egg yolks with the sugar until white. Carefully mix the chocolate-butter mixture in. Add the ground almonds.
Beat the egg whites and carefully incorporate them to the rest of the mixture.
When the mixture is smooth, distribute it evenly onto the mold (it should be about 1,5-2cm thick) and place it into the oven for about 25 minutes.