Preheat oven to 350°F / 177°C / Gas Mark 4
In a medium bowl, sift together the flour, cocoa powder, baking powder and salt. Set aside.
Melt chopped chocolate in a small glass bowl in 30 second increments in the microwave. Stir until smooth and set aside to cool to room temperature.
In the bowl of a stand mixer, beat butter and sugars together on medium speed until light and fluffy, about 4 - 5 minutes.
Add eggs, one at a time and beat until combined. Scrape down bowl and run mixer for another 30 seconds to incorporate everything until uniform.
Pour in melted chocolate and mix until combined.
Add flour mixture and mix until a stiff dough forms, about 2 minutes.
Add mint chips into mixer bowl and "pulse" just to get it slightly mixed. Remove the bowl from the mixer and finish folding in chips by hand.
Divide dough evenly. Then form 2 loaves of dough roughly 3" wide by 3/4" thick.
Bake for 30 minutes, then remove from oven and let cool for 15 minutes. Reduce oven temperature to 325°F / 165°C / Gas Mark 3
Gently slice each loaf into 1/2" slices and place each slice on its side (cut side down) and return to the oven for an additional 15 - 20 minutes.
Cool completely, then drizzle with melted chocolate and top with sprinkles, if desired.