In a small saucepan or a microwave-safe bowl, combine chocolate and butter. Heat until butter is melted and remove from heat. Stir until the chocolate is melted and the mixture is smooth. Set aside.

In a large mixing bowl, beat eggs, sugar, vanilla extract and espresso powder. Stir in the chocolate mixture, baking powder and salt. Mix until combined.

Finally add the flour and mix until combined. Cover and put the bowl in the fridge for 2~3 hours or overnight. This will firm up the dough considerably.

Preheat the oven to 325°F. Line 2 baking sheets with parchment paper.

Place the confectioners sugar in a bowl. Using a small scoop or a spoon, scoop out heaping teaspoon-sized portions of the dough (about 1¼" in diameter). Drop them into the bowl of sugar.

After adding about 4~5 cookie balls, shake the bowl to coat the balls with sugar. Don't try to add more then 4~5 in the bowl -- they might tend to stick each other instead of being evenly coated with sugar. Place the cookies on the prepared baking sheet, leaving about 1½" between them.

Bake the cookies for 10~12 minutes. Switch the baking sheets midway through baking. Cookies flatten out and acquire the distinctive 'streaked' appearance while baking.

Remove from the oven and let the cookies cool on a wire rack. Store in an airtight container at room temperature for up to 1 week. Freeze for longer storage.