1. Heat oven to 350°F. Unwrap and coarsely chop peppermint patties; set aside. Stir together cookie crumbs, 1/4 cup cocoa, 1/2 cup powdered sugar and melted butter. Press firmly on bottom of ungreased 13x9x2-inch baking pan.
2. Beat sweetened condensed milk, flour, remaining 1/4 cup cocoa, egg, vanilla and baking powder in large bowl until well blended. Spread evenly over prepared crust. Set aside 2 tablespoons peppermint pattie pieces; sprinkle remaining pieces over filling.
3. Bake 18 to 23 minutes or until set. Cool completely in pan on wire rack. Sprinkle cooled bars with additional powdered sugar.
4. Place reserved peppermint patties and water in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds; stir. If necessary, microwave an additional 10 seconds at a time, stirring after each heating, until patties are melted and smooth when stirred. Immediately drizzle over bars. Allow drizzle to set. Cut into bars. Makes about 24 bars.