Directions

1. 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.)
2. 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.
3. 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.