Position one rack in the center of the oven and one rack in the bottom third of the oven and preheat to 300 degrees F. Line 2 large rimmed baking sheets with parchment paper.
Combine the chocolate and butter in a small bowl and place over a saucepan of barely simmering water. Stir occasionally until the chocolate has melted and the mixture is smooth. Cool for 10 minutes.
Whisk together the flour, cocoa, baking powder and salt in a medium bowl. Whisk the sugar, eggs, 2 tablespoons water and vanilla to blend in a separate large bowl. Stir in the dry ingredients and fold in the cooled chocolate using a rubber spatula. Fold in chocolate mint pieces. Chill dough to firm slightly, 10 minutes.
Using a 1/4 cup to measure, drop 6 mounds of batter onto each baking sheet, spacing apart. Put one sheet on the center rack in the oven, and one on the bottom rack.
Bake cookies 9 minutes. Switch the baking sheets (top to bottom) and bake the cookies until slightly puffed and dry looking with some small cracks on top, 9 to 10 minutes longer. Cool completely on the baking sheets (cookies may deflate slightly as they cool).