Directions

1. Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper.
2. Using a mixer with the whisk attachment, beat yolks and 1/2 cup granulated sugar until pale and stiff, about 3 minutes. Add espresso powder, and beat for 2 minutes. Add flour, and beat until just combined. (Mixture will be very thick.)
3. In a clean bowl, using a clean whisk attachment, beat whites and salt on medium speed until foamy. With the machine running, add remaining 1/2 cup granulated sugar in a slow stream, beating until stiff peaks form, about 5 minutes. Fold whites into yolk mixture in 3 additions until well combined.
4. Using a pastry bag fitted with a 1/2-inch plain tip (such as Ateco #806), pipe batter onto baking sheets into 60 lines that are 2 inches long and 1 inch wide, spacing 1 inch apart. Dust with cocoa.
5. Bake, rotating sheets halfway through, until firm, 12 to 14 minutes. Let cool completely on parchment on wire racks. (Cookies can be stored for up to 3 days.)
6. Stir together mascarpone, confectioners' sugar, liqueur, and vanilla until well combined. Cover, and refrigerate for 15 minutes (or up to 3 days).
7. When ready to serve, brush the cookies' flat sides with melted chocolate. Refrigerate, flat side up, until chocolate is firm, about 10 minutes.
8. Spread a generous 1/2 teaspoon mascarpone filling over chocolate side of half of the cookies. Sandwich with remaining cookies. Serve immediately.