For the Cakes, preheat oven and prepare cake batter as directed on package. Divide batter evenly into 2 bowls (about 1 1/2 cups of batter in each bowl). Stir red food color and vanilla into first bowl. Stir green food color into second bowl. Pour the red and green batter into separate greased 13x9-inch baking pans. Gently tap on counter to remove any air bubbles. (If you do not have 2 13x9-inch pans, bake one color cake first, then wash and reuse the pan to bake the second cake.)
Bake 6 minutes. Carefully remove pans from oven and tap on counter to remove any air bubbles. Return pans to oven and bake 6 to 9 minutes longer or until toothpick inserted into center comes out clean. Cool in pans 10 minutes. Invert cakes onto wire rack. Cool completely. Refrigerate cakes until ready to assemble.
Meanwhile for the Buttercream, beat butter in large bowl with electric mixer or in stand mixer with whisk attachment on medium-high speed until light and fluffy, about 3 minutes. Add vanilla; mix well. Gradually beat in confectioners’ sugar on low speed, scraping sides and bottom of bowl after each addition. Add milk and salt; beat on medium-high speed until light and fluffy, at least 3 minutes. If frosting is too thick to spread, gradually beat in additional milk.
If not using frosting immediately, refrigerate in airtight container up to 2 weeks. Rewhip before using.
When ready to assemble, place cooled green cake on a large shallow parchment paper-lined baking pan. Spread top of cake with buttercream, smoothing out the frosting so that it is flat and even. Top with red cake layer. (You may need to refrigerate the assembled cake until it is firm. If cake is too soft, it will be difficult to cut in the next step.)
Cut 8 circles from assembled cake with a 2 1/2-inch round cookie cutter to make 8 mini cakes. Place mini cakes on wire rack on top of parchment paper-lined tray or large shallow baking pan.
Melt chocolate as directed on package. Pour chocolate over each mini cake, smoothing with an offset spatula if needed. Transfer cakes to separate parchment paper-lined pan. Let stand at room temperature or refrigerate 30 minutes or until chocolate is set.
Test Kitchen Tip: To use semi-sweet chocolate chips in place of candy coating, mix 16 ounces chocolate chips and 1 tablespoon shortening in large microwavable bowl. Microwave on HIGH 1 minute; stir. Microwave an additional 30 seconds at a time until chocolate is smooth and melted. Pour over cakes as directed in recipe.