Place the semisweet and unsweetened chocolate in the top of a double boiler or in a bowl set over a saucepan of hot water over low heat (the bottom of the bowl should not touch the water). Stir occasionally until the chocolate melts; remove from the heat. Learn different techniques for How To Melt Chocolate.
When the chocolate is smooth, stir in the butter and sugar until smooth. Add the eggs, flour, baking powder and cocoa. Beat with an electric mixer at medium-high speed until the mixture is pale and has a thick, mousse-like consistency, about 5 minutes.
Fill the ramekins 1/2 full and cover each with plastic wrap. Freeze for at least 3 hours. NOTE: The ramekins can be filled and frozen up to 3 days in advance.
Just before serving time, preheat the oven to 375 degrees F. Place oven rack in the center of the oven.
Bake the cold desserts until the outer edges of the tops are set, but the centers are still moist and shiny, usually 10 to 11 minutes. Remove from oven.
To serve, invert each hot portion onto a serving plate and serve warm, with a small scoop of the vanilla ice cream or a spoonful of vanilla-flavored whipped cream. Alternately, serve directly from the ramekins.
Makes 6 servings.
Thomas Keller's Molten Chocolate Tart Recipe: https://whatscookingamerica.net/chocmoltentart.htm