Instructions

• Make the melted chocolate base for the dough: place 7 ounces of the semisweet chopped chocolate and all 3 ounces of the unsweetened chopped chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring well between intervals, until the chocolate is fully melted and smooth. Let cool slightly (a few minutes) so it is warm but not hot.
• Whisk together the 3 eggs, 1 cup plus 2 tablespoons sugar, 1 teaspoon instant espresso powder, and 2 teaspoons vanilla extract in a medium bowl until combined. Whisk this mixture into the slightly cooled melted chocolate until smooth and even.
• Combine the dry ingredients: in a separate bowl stir together 6 tablespoons flour, 3/4 teaspoon baking powder, and 1/4 teaspoon kosher salt. Fold the dry ingredients into the chocolate-egg mixture until just incorporated.
• Stir in the 1 1/2 cups semisweet chocolate chips until evenly distributed through the dough. Cover the bowl and chill the dough in the refrigerator until firm enough to shape, about 1 hour.
• While the dough chills, make the chocolate filling (ganache-style) using the remaining chocolate and the butter: reserve the remaining 3 ounces of semisweet chopped chocolate. Put the reserved 3 ounces semisweet chocolate and the 1/4 cup butter in a microwave-safe bowl. Microwave in 20- to 30-second intervals, stirring between intervals, until the chocolate and butter are melted and the mixture is smooth. Let cool, then cover and chill, stirring once or twice, until the filling is thick enough to spread or pipe, about 2 hours. (If the filling becomes too firm to spread, warm briefly in the microwave in 5–10 second bursts, stirring after each.)
• Preheat the oven to 350°F. Line two large baking sheets with parchment paper.
• Shape the cookies: scoop about 2 tablespoons of chilled dough and roll into a round between your palms. Place rounds on the prepared baking sheets about 1 inch apart. Gently press each round to flatten to about 1/2-inch thickness.
• Bake the cookies for about 10 minutes, until the tops lose their wet shine but are not overbaked. (Oven times vary; check at 8 minutes if your oven runs hot.)
• Let the cookies cool on the baking sheets set on wire racks until firm enough to handle.
• Assemble the sandwiches: spread or pipe a generous tablespoon (or to taste) of the chilled chocolate filling onto the flat side of half the cookies, then top with the remaining cookies, flat side down, to form sandwiches. If the filling is too firm to spread, warm briefly in the microwave in 5–10 second bursts and stir before using.