Preheat oven to 350 degrees F. Line an 8"x8" baking pan with parchment paper and set aside.
In the bowl of a mixer, cream together the butter and brown sugar until it becomes light and fluffy, about 3 minutes. Add the egg and vanilla extract and continue to beat until incorporated.
In a separate bowl combine the flour, baking powder and salt and sift together. Add half of the dry ingredients to the bowl with the wet ingredients and mix together. Scrape down the sides of the bowl and add remaining half of the dry ingredients. Mix again. Fold in the chocolate chunks, leaving a few for sprinkling on top.
Spread batter into the prepared pan and bake for 20 to 25 minutes, or until the center is set. Remove from the oven and let cool. Slice into 9 pieces and enjoy!