1. Heat oven to 350°F. Line 13x9x2-inch baking pan with foil, extending foil beyond sides. Grease foil. remove wrappers from peanut butter cups. Cut each peanut butter cup into 4 pieces; set aside.
2. Place butter in large microwave-safe bowl. Microwave at HIGH (100%) 1 minute or until butter is melted. Stir in 3/4 cup cocoa until smooth. Add sugar, 3 eggs and 1 teaspoon vanilla; blend well. Blend in flour; spread in prepared pan. Bake 15 minutes.
3. Meanwhile, combine sweetened condensed milk, remaining 1/2 cup cocoa, remaining egg and remaining 1-1/2 teaspoons vanilla. Pour over baked layer. Return to oven; bake 20 to 25 minutes or until set.
4. Place marshmallow creme by heaping teaspoonfuls over hot surface. Allow to soften about 5 minutes; carefully spread over surface. Immediately sprinkle peanut butter cup pieces and peanuts over marshmallow. Cool completely in pan on wire rack. Refrigerate until thoroughly chilled.
5. Lift dessert from pan using foil as handles; place on cutting board. Cut into squares using wet knife. To serve, gently heat in microwave at MEDIUM about 10 seconds or 200°F. oven until marshmallow softens. (Dessert may also be allowed to soften at room temperature until ready to serve.) Cover; refrigerate leftovers. About 20 servings.