Adjust oven rack to middle position and heat to 375 degrees. Line a 13-by-9-inch metal baking pan with one sheet of foil, allowing 2 inches of foil to hang over each end of pan. Butter foil (except overhang).
Whisk together flour, cocoa powder, baking soda, and salt in a small bowl. In the bowl of a standing or electric mixer, cream butter and brown sugar until light and fluffy. Beat in egg until combined. Reduce speed to low, then mix in flour mixture until just combined. Stir in chocolate chips and candy.
Spread dough evenly in pan and bake until puffed and beginning to pull away from sides of pan, about 20 minutes. Cool completely in pan on a rack. Then, lifting with foil ends, transfer to a cutting board. Cut into bars (about 2¼ by ½ inch each) and lift them off foil with a spatula.
Store in an airtight container at room temperature up to 3 days.