Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

In a small bowl, sift together flour, espresso powder, baking powder, and salt. Set aside.

In a saucepan over low heat, melt semi-sweet baking chips, unsweetened baking chocolate, and butter until smooth. Remove from heat and let cool for 5-10 minutes.

In a large bowl, beat eggs, granulated sugar, and vanilla extract on medium-high speed until pale and fluffy, about 5-7 minutes.

Reduce mixer speed to low and gradually add the cooled chocolate mixture, mixing until fully incorporated.

Add the flour mixture and beat on low until just combined.

Drop tablespoonfuls of dough onto prepared baking sheets, spacing them about 2 inches apart.

Bake for 10 minutes, or until the edges are set and the tops spring back when touched.

Allow cookies to cool on baking sheets for 3 minutes, then transfer to wire racks to cool completely.

For the filling, dissolve espresso powder in warm coffee.

In a bowl, beat mascarpone cheese, butter, and confectioners’ sugar until smooth. Add the coffee mixture and beat until combined.

Once cookies have cooled, spread or pipe the filling onto the flat side of one cookie and sandwich with another.

Refrigerate the assembled cookies for at least 1 hour before serving.​