1. Preheat oven to 350°F(175°C). Select a 10-inch cake pan with straight sides. Prepare the pan by rubbing the bottom and sides of the pan with about 1/4 teaspoon of butter. Add about 1/2 teaspoon of flour to the pan. Shake the pan to coat with an even layer of flour; discard excess flour. Line the bottom of the cake pan with a greased 10-inch parchment circle. Line the inner sides of the cake pan with a greased 4-inch wide strip of parchment paper, creating a "collar" that extends about 2 inches above the edge of the pan.
2. Combine chocolate and 1/2 cup of butter in a double boiler and cook, stirring constantly, until melted. Using a wire whisk, stir in cocoa; keep warm.
3. Combine egg whites and sugar and beat mixture until soft peaks form.
4. Stir egg yolks into the chocolate mixture.
5. Fold egg white mixture into chocolate mixture.
6. Pour mixture into cake pan and bake in center of oven for 40-45 minutes. The edges will appear cooked while the center will still be moist.
7. Remove the cake from the oven and remove parchment "collar". Place a plate upside down on top of the cake pan; invert cake onto plate. Remove the parchment round. Let cool, but do not refrigerate.
8. Combine cream, honey, and lemon zest. Whip until soft peaks form. Slice cake and garnish with whipped cream and mint sprigs.