Preparation

Make the cookies

• In the top of a double boiler, melt the chocolate and butter over barely simmering water. As soon as it’s melted, stir gently with a spoon and remove from the heat.
• Meanwhile, combine the eggs, granulated sugar, and brown sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium-high speed until thick and pale yellow, about 5 minutes. Reduce the speed to low. Add the vanilla and melted chocolate-butter mixture and mix just until blended.
• In a medium bowl, combine the flour, baking powder, and chocolate chips. Add to the egg-chocolate mixture, and mix on low, scraping down the sides of the bowl as necessary, until incorporated, 30 seconds. Cover and chill until firm, 2 hours or overnight.
• Preheat the oven to 325°F. Line a baking sheet with parchment paper.
• 删除cookie窦gh from the refrigerator. Roll or scoop the dough into 1-tsp.-size balls, and place them 2 inches apart on the prepared baking sheet. Bake until barely set and the surface is just starting to crack, 6 to 8 minutes. Transfer to a wire rack and let cool completely before filling.

Make the filling and fill the cookies

• In the top of a double boiler, melt the chocolate over barely simmering water until smooth. Remove from the heat, then slowly add the cream in a steady stream, whisking continuously. Pour in the espresso and whisk until smooth and glossy.
• Turn half of the cookies over, and pipe or spoon just under 1 tsp. of ganache onto each. Top with remaining halves, tops up, to form a “sandwich” and chill thoroughly to set the ganache. Store in an airtight container in the refrigerator for up to 4 days.