Preheat oven to 350 degrees. Spray your madeleine pan with non-stick cooking spray.
In a microwave-safe bowl, microwave the butter and semi-sweet baking chocolate for 30 seconds. Stir the mixture. Repeat 2 additional times. Let cool while you beat the sugar and eggs.
In a medium bowl, add in the sugar, eggs, and vanilla extract. Beat on high for about 5 minutes. The mixture will have volume, be pale yellow, and thick.
Once the egg mixture is beat, sift the cocoa powder, flour, and baking powder into it. Carefully fold it.
Add the butter and chocolate mixture and mint chips into the the batter and stir.
Place the batter into a disposable piping bag or large storage bag. Cut a small hole in the corner of the bag and pipe the batter into the pan. Don't overfill the pan. I put about 1 ½ to 2 tablespoons in each madeleine mold.
Place the pan in the oven and bake for 9 - 10 minutes until the madeleines are done. Be sure the pan is well coated with oil or non-stick cooking spray otherwise they may stick.