Preheat oven to 350° (175°C). Grease an 8-inch square baking pan.
Whisk together flour, baking powder, and salt. Set aside.
Beat butter and brown sugar until fluffy. Add egg and vanilla, and mix well. Gradually add flour mixture, mixing just until combined. Stir in chocolate chunks.
Spread batter evenly in prepared pan. Sprinkle coarse salt over top of batter. Bake for 22-25 minutes, or until edges are browned and a pick inserted in the center comes out clean.