Place the chocolate and butter in a small saucepan and heat 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, do not skip this step
Preheat the oven to 160°C. Lightly grease a couple of baking sheets, or line them with parchment.
Put powdered sugar into a shallow bowl. Using a cookie scooper or a teaspoon, or your fingers, scoop out heaping teaspoon-sized portions of the dough. 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 a couple cms between them
Bake the cookies for 10 to 12 minutes. 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