In the top of a double boiler, melt chocolate and butter, stirring frequently, until smooth. Let cool to room temperature.

Preheat oven to 350°F (180°C). Line baking sheets with parchment paper.

In the work bowl of a food processor, pulse dark chocolate-covered coffee beans until most of beans are finely ground; set aside.

In the bowl of a stand mixer fitted with the whisk attachment, beat eggs and sugar at high speed until very thick and pale, 5 to 6 minutes. With mixer on low speed, add cooled chocolate mixture in three additions, beating until combined after each addition.

In a medium bowl, whisk together flour, baking powder, and kosher salt. With mixer on low speed, gradually add flour mixture to egg mixture, beating just until combined, stopping to scrape sides of bowl. Beat in cooled espresso and vanilla. Fold in ground chocolate-covered coffee beans.

Drop batter by 2 tablespoonfuls (35 to 40 grams) about 2 inches apart onto prepared pans. (Dough will be very loose, similar to brownie batter. Use a small spatula to cleanly drop batter from tablespoon scoop.)

Bake until puffed and cracked, 12 to 13 minutes. Let cool completely on pans. Store in airtight containers between layers of parchment paper at room temperature until ready to serve.

To serve, spread dulce de leche onto cookies. Sprinkle with flaked sea salt and crushed coffee beans, if desired.