Preheat the oven to 350 degrees. Butter and flour a 10-inch round cake pan. For the cake: Melt the chocolate and butter in a double boiler (or in a bowl or smaller saucepan placed over a saucepan of boiling water). Set aside to cool. Beat the egg yolks until light and fluffy. Slowly add the sugar and continue beating until mixture is pale yellow. Fold in the melted chocolate. Sift the flour over the chocolate mixture until it just disappears. In a separate bowl, whisk the egg whites until soft peaks form. Fold egg whites into the chocolate mixture in 2 parts. Pour the batter into the prepared pan and tap on the counter to remove air bubbles. Bake for 20 to 25 minutes, until a toothpick comes out clean (see Cook's Note). For the ganache: Place the chopped chocolate in a mixing bowl. Place the cream in a saucepan and bring to a boil. Add the hot cream to the chocolate and stir until all lumps are gone. Let the ganache stand until it reaches room temperature, then pour it over the cooled cake and spread with a spatula.
For a thicker cake, double the recipe and bake for 35 minutes.