Mascarpone-Filled Brownie Sandwiches

In a small heavy saucepan heat and stir chopped semisweet chocolate, unsweetened chocolate, and butter over low heat until melted and smooth; set aside to cool slightly. In a small bowl stir together the flour, baking powder, salt, and espresso powder; set aside.

In a medium mixing bowl beat eggs, granulated sugar, and 1 teaspoon vanilla with an electric mixer on high speed for 4 minutes or until thick and light yellow in color. Fold in melted chocolate mixture. Fold in flour mixture until combined. Fold in semisweet chocolate pieces. Let batter stand for 10 minutes (batter will thicken as it stands).

Preheat oven to 375°F. Line two cookie sheets with parchment paper. Drop batter by slightly rounded tablespoons 2 inches apart onto the prepared cookie sheets. Bake about 8 minutes or until set and tops are crackled. Cool on cookie sheets for 1 minute. Transfer to a wire rack and let cool.

For filling, in a small bowl stir together the mascarpone cheese, powdered sugar, whipping cream, and 1/2 teaspoon vanilla until smooth. Spread about 1 tablespoon filling each on the flat sides (bottoms) of half of the cookies. Press the flat sides of the remaining cookies against the filling.