Whisk flour, espresso, and salt in a bowl. Put butter and sugar in the bowl of an electric mixer fitted with the paddle, and mix on medium-high speed until pale and fluffy. Mix in vanilla. Add flour mixture; mix on medium-low until dough comes together. Shape into a disk, and wrap in plastic. Refrigerate 30 minutes.Advertisement
Preheat oven to 350 degrees. Let dough stand at room temperature 10 minutes. Roll between sheets of parchment to 1/8 inch thick. Cut out rounds with a 1 1/8-inch cutter; space 1/2 inch apart on baking sheets lined with parchment. Reroll scraps; cut out. Freeze 10 minutes.
Bake until set but not browned, about 9 minutes. Let cool on sheets 5 minutes. Transfer to wire racks; cool completely.
Bring cream to a simmer in a saucepan. Pour over chocolate in a bowl; stir until smooth. Press plastic wrap onto surface; refrigerate at least 4 hours (up to overnight). Whisk to soft peaks before using.
Transfer filling to a pastry bag fitted with a 1/4-inch plain round tip. Pipe about 1 teaspoon filling on bottoms of half the cookies; sandwich with remaining cookies. Dust with cocoa and sugar.
Cookies are best eaten day of but can be refrigerated in an airtight container up to 1 day.