PREHEAT oven according to cake mix package directions. Grease and flour two 9-inch-round cake pans.
PREPARE cake mix according to package directions. Pour half of cake batter into prepared pans. Stir Nesquik and milk into remaining batter. Stir in morsels. Spoon chocolate batter over plain batter in each pan; gently cut through batters to marble.
BAKE according to cake mix package directions. Cool in pans on wire racks for 10 minutes; remove to wire racks to cool completely. Place one cake on serving platter; spread about 1/2 cup Butter Frosting over top of cake. Top with remaining cake; frost with remaining Butter Frosting.
DECORATE as desired following decorating tips below.
FOR BUTTER FROSTING:
BEAT butter in small mixer bowl until fluffy. Gradually beat in 2 cups sugar. Slowly beat in 1/4 cup milk and vanilla extract. Gradually beat in remaining sugar. Beat in additional milk until of spreading consistency.
• For triangle and animal cookie cutouts, prepare your favorite sugar-cookie cutout dough; roll out to 1/8 inch thickness. Cut out three 4 x 2-inch rectangles; cut each rectangle diagonally in two to form triangles. Reroll dough. Using cookie cutters, cut out six to eight animal shapes (about 2 1/2 inches tall). Bake and cool. Glaze animal shapes with yellow-tinted powdered sugar icing.
• Prepare another batch of Butter Frosting. Use food colorings to tint one fourth of the frosting blue, one fourth of the frosting yellow, one fourth of the frosting purple, and one fourth of the frosting pink.
• Using a decorating bag fitted with a coupler and a medium leaf tip, pipe the blue frosting to make six lines coming out from center on the top of the frosted cake. Top each blue line with a triangle-shaped cookie cutout, with short sides of triangles meeting in center. Top each triangle cookie cutout with a round mint, using frosting to attach. Affix a gumball on top of mint with a little frosting.
• Using a decorating bag fitted with a small star tip, pipe yellow frosting around edge of top cake layer. Using a decorating bag fitted with a small leaf tip, pipe purple frosting over top edge of each triangle-shaped cookie cutout. Using a decorating bag fitted with a small round tip, pipe pink frosting into a line of dots on top of purple frosting.
• Refit the decorating bag filled with blue frosting with a small star tip (if decorating bag doesn’t have a coupler, fit a new bag with small star tip and put blue frosting in bag). Pipe blue frosting around bottom of cake. Pipe blue, pink, and purple frosting onto glazed animal-shaped cookie cutouts to add decorative accents. Use a little frosting on the back of each animal-shaped cookie cutout to affix cutouts to sides of cake.