Place the chocolate and butter in a small saucepan or microwave-safe bowl, and heat or microwave until the butter melts. Remove it from the heat, and stir until the chocolate melts and the mixture is smooth.
In a separate bowl, beat together the sugar, eggs, vanilla and espresso powder. Stir in the chocolate mixture, baking powder and salt, then the flour. Chill the dough for 2 to 3 hours, or overnight; it'll firm up considerably. You need to do that since the preparation will be to smooth if you cook it to quickly.
Preheat the oven to 325°F. Lightly grease a couple of baking sheets, or line them with parchment.
Shape the cookies: Put a cup of confectioners' sugar into a bowl. Using a spoon, or your fingers, scoop out a teaspoon-sized portions of the dough; they should be roughly 1 1/4" in diameter. Drop the dough balls into the confectioners' sugar as you go.
Once about five or six are in the bowl, shake and toss the bowl to coat the balls with the sugar. (If you try to do this with too many balls at a time, they'll just stick together.) Place the cookies on the prepared baking sheets, leaving about 1 1/2" between them.
Bake the cookies for 10 to 12 minutes, switching the position of the pans (top to bottom, and front to back) midway through the baking time. As the cookies bake, they'll flatten out and acquire their distinctive "streaked" appearance.
Remove the cookies from the oven, and allow them to cool on a rack. Store them at room temperature, well wrapped, for several days; freeze for longer storage.