1. Heat oven to 350°F. Grease bottom and sides of 13x9x2-inch baking pan.
2. Stir together crumbs, 1/2 cup cocoa and sugar; cut in 3/4 cup (1-1/2 sticks) butter with pastry blender or 2 knives until mixture is crumbly. Press evenly on bottom and 1/2-inch up sides of prepared pan. Sprinkle marshmallows and coconut over chocolate layer.
3. Combine sweetened condensed milk, remaining 1/2 cup cocoa and remaining 1/4 cup (1/2 stick) butter in microwave-safe bowl. Microwave at HIGH (100%) 1 to 1-1/2 minutes or until mixture is smooth when stirred; evenly pour over coconut. Sprinkle evenly with nuts and pretzel pieces; press down firmly.
4. Bake 25 to 30 minutes or just until bubbly on top. Cool completely; cut into bars. Store leftover bars tightly covered at room temperature. Makes 24 to 36 bars.