Preheat oven to 325°F and line two baking sheets with parchment paper or silicone liners.
In a small saucepan over medium-low heat, melt 4 tablespoons of butter and chocolate chips until completely smooth. Set aside to cool for 10 minutes.
Meanwhile, in a large bowl whisk brown sugar, eggs and peppermint extract. Set aside.
In a separate bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt.
Pour the melted chocolate mixture into the eggs and whisk until combined. Add the flour mixture and stir until combined. Stir in chocolate chips. Batter will be thick.
Portion the dough into generous 2 teaspoon sized balls and place 2 inches apart on prepared baking sheets. Dampen your palm, and lightly press down on each mound of dough to flatten slightly. Bake for 7-10 minutes, until the edges are set and the centers are puffed and still appear slightly underdone (they will continue to cook as they cool). Place baking sheets on cooling racks and cool completely.