Preheat oven to 350 degrees F. Line an 8×8 baking pan with parchment paper. Set aside.
In a large bowl, mix together melted butter and brown sugar until smooth. Add egg and vanilla and mix until light and creamy. Slowly add flour and baking powder and mix until just combined. Add chocolate chips and stir until incorporated. Pour batter into prepared pan. Bake for 20-25 minutes. Take out of oven and sprinkle chopped chocolate on top. Return to oven and bake an additional 2 minutes. Remove to cooling rack and let cool. Once cooled, store in airtight container.
