Heat oven to 350°F. Line 13x9-inch pan with foil, extending foil over sides of pan. Grease foil. In large bowl, combine brownie mix, oil, water and eggs; beat 50 strokes with spoon. Stir in 1 cup candy baking pieces. Spread batter in greased foil-lined pan.
Bake at 350°F. for 31 to 33 minutes or until set. DO NOT OVERBAKE. Cool 30 minutes or until completely cooled. Freeze brownies 30 minutes.
Using foil, lift brownies from pan; place on cutting board. To cut tree shape from brownies, start at center of 1 short side and make 2 diagonal cuts to corners of opposite short side, forming a triangular piece in center. (See diagram.)
Add green food color to can of vanilla frosting; stir to combine. Place 2 brownie side pieces together on foil-lined serving tray to form tree shape. Spread 1/3 cup frosting over top of tree shape. Top with whole tree shape; trim if necessary to line up edges.
Frost sides and top of brownie tree with remaining frosting. Sprinkle 1/3 to 1/2 cup candy pieces decoratively over top of tree. Decorate as desired. If desired, place 1 junior-sized candy bar, unwrapped, at base of brownie tree to form trunk.