For the cake: Melt the chocolate and butter together. Cool slightly. Meanwhile, in a separate bowl, whisk together the yolks and eggs then mix in the powdered sugar. Whisk this into the chocolate mixture then whisk in the flour. Fill 12 greased 4-ounce foil tins with the batter and refrigerate until ready to bake, or bake immediately. Bake in a preheated 450 degree F oven for 5 to 6 minutes from cold, or 4 to 5 minutes from room temperature. Turn out of tins immediately onto the plate.
For the ganache: Heat the cream in a saucepan until boiling. Place the chopped chocolate in a bowl. Whisking well, pour the hot cream over the chocolate to melt it. Cover and chill overnight. The next day, whip it up like whipped cream until soft peaks. Place a dollop of it on the cake before serving.
For the mint sauce: In the top half of a double boiler, combine the 2 chocolates over simmering water. Stir constantly until melted, then whisk in the syrup, water, and mint extract without removing the double boiler from the heat. Whisk until smooth and shiny. (The sauce can be made up to 48 hours in advance and refrigerated. To rewarm, stir over low heat or heat in a microwave.) Drizzle this over the cake and plate just before serving.