1. Heat oven to 350°F. Grease 13x9x2-inch baking pan. Unwrap and coarsely chop peppermint patties; set aside.
2. Stir together cocoa and baking soda in large bowl; stir in 1/3 cup butter. Add boiling water; stir until mixture thickens. Stir in sugar, eggs and remaining 1/3 cup butter; stir until smooth. Add flour, vanilla and salt; blend completely. Stir in peppermint pattie pieces. Spread in prepared pan.
3. Bake 35 to 40 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack. Sprinkle with powdered sugar; cut into bars. Garnish with peppermint pattie pieces, if desired. Makes 24 to 36 brownies.