Place 1 oven rack in the center of the oven and 1 oven rack in the bottom third and preheat the oven to 300 degrees F. Line 2 large rimmed baking sheets with parchment paper. Set aside.
In a small bowl, combine the bittersweet chocolate and butter. Place the bowl over a pan of barely simmering water and stir occasionally until the chocolate has melted and the mixture is smooth.
In a medium bowl, sift the flour, cocoa, baking powder and salt. In a large bowl, whisk together the sugar, eggs, 2 tablespoons water and vanilla extract. Gradually add the dry ingredients. Fold in the cooled chocolate. Stir in the chocolate mint pieces. Chill the dough 10 minutes to firm slightly. Using 1/4 cup measuring cup, drop 6 mounds of batter onto each prepared sheet, spacing evenly apart.
Bake the cookies 9 minutes. Reverse the baking sheets (top to bottom) and continue baking until the cookies are slightly puffed and dry looking with some small cracks on top, 9 to 10 minutes longer. Cool the cookies completely on sheets (they may deflate slightly as they cool).