Preheat the oven to 350 degrees F, with the oven racks in the upper-middle and lower-middle positions. Line 2 large baking sheets with parchment paper.

In a medium bowl, melt semi-sweet chocolate. This can be done by placing the bowl over a pot of simmering water (for a make-shift double-boiler) or in the microwave at 50% power. Stir and set aside to cool.

In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt. In the bowl of a stand mixer (or in a large bowl, using a hand electric mixer), beat butter, brown sugar, and granulated sugar at medium at speed until the mixture is light and fluffy, 3 to 5 minutes. Add one egg a time to the butter mixture, beating after each addition. Add vanilla extract and beat until combined. Add the cooled melted chocolate and beat until well combined.

Reduce the mixer speed to low and gradually add the flour mixture. Mix until just combined. Stir in mint-chocolate baking bits. Cover the bowl with plastic wrap and let the mixture sit at room temperature for 15 to 20 minutes, or until the mixture is easy to scoop and fudge-like.

Using 3 tablespoons of dough for each cookie, roll the dough into balls and place on the prepared baking sheets, 2 inches apart. Place the baking sheets in the oven and bake for 10 to 12 minutes, switching the position of the sheets and rotating them back to front halfway through baking. The cookies are baked when the edges are set, but the center are still soft. The cookies will still look a little raw in between the cracks, but are done.

Let the cookies cool on the baking sheets for 10 minutes, then remove to a wire rack. Repeat with the rest of the dough, using cooled cookie sheets. Serve the cookies warm or at room temperature.