1. Heat oven to 350°F. Grease and flour two 9-inch round baking pans.
2. Stir together sugar, flour, cocoa, baking powder, baking soda and salt in large bowl. Add eggs, milk, oil and vanilla; beat on medium speed of mixer 2 minutes. Stir in boiling water (batter will be thin). Pour batter into prepared pans.
3. Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire rack. Cool completely.
4. Set aside 1/2 cup frosting. Tint remaining frosting to desired shade of orange with red and yellow food color.
5. Coarsely chop 1 cup mini peanut butter cups; set aside. Place first cake layer on serving plate; frost top of layer with about 3/4 cup frosting. Top with chopped peanut butter cups. Add second cake layer; complete frosting cake.
6. Add cocoa to reserved frosting; place in heavy duty (freezer) resealable food storage bag. Cut off tip of bag about 1/4 inch from one corner. Draw leafless tree and branches onto top of cake by piping chocolate frosting in desired design. Add remaining mini peanut butter cups as few remaining leaves on tree and as leaves that have already fallen to the ground. 10 to 12 servings.