Place two-thirds of the chocolate and all the butter in the top of a double boiler to melt over medium heat. Be sure not to let the water spatter, turning the heat down if necessary. Place eggs, vanilla, and sugar in a large bowl and using a stand or hand mixer, beat until light and creamy - think the color "buff"- about 10 minutes. Stir in all remaining ingredients: the flour and baking powder mixture, the stirred chocolate-butter mixture, and the remaining chopped chocolate. Let stand for 10 minutes, and preheat oven to 350 degrees(F).

Spoon tablespoonfuls onto parchment-lined baking sheets. Bake for 8 minutes or so, until tops are puffed and cracked. Allow to cool completely on trays before making into sandwiches. To make the filling, stir together mascarpone, sugar, and chopped berries. This can be done ahead of time and re-refrigerated. For the sandwiches, spread a nice layer of the mascarpone creme on one cookie, and sandwich with another.