Trim blondie to 6-inch square; repeat with brownie. Cut out small bones from scraps with dog-bone-shape cutter.
Coat a large dog-shape cutter with cooking spray. Cut out a dog from center of each 6-inch square. Carefully place brownie dog into cutout in blondie, and blondie dog into cutout in brownie. Spread brownie with frosting. Top with blondie. Refrigerate until set, 20 minutes. Serve with ice cream and dog bones.