Preheat oven to 375F. Place rack in middle position of oven. Line a 13-9 inch metal baking pan with 1 sheet of foil, allowing 2 inches of foil to hang over each end of pan. Butter foil, except for overhang.
Whisk together flour, cocoa, soda and salt in a small bowl.
Beat together butter and brown sugar in a large bowl with an electric mixer at high speed until pale and fluffy, about 3 minutes. Beat in egg until combined.
Reduce speed to low and mix in flour mixture until just combined. Stir in chocolate chips and peppermint candy pieces.
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, then, lifting with foil, transfer to a cutting board. Cut into bars and lift off foil with spatula.