Melt the butter and chopped-up dark chocolate bar in the microwave. Beat the sugars into the melted chocolate mixture using a hand mixer on medium speed.

Pour the brownie batter into a parchment paper-lined 8×8-inch baking pan. Smooth the batter out and then sprinkle on chocolate chips.

You can store these flourless brownies in an airtight container for 1-2 days at room temperature. These will also stay good for up to 3-4 days in the refrigerator.