- Preheat oven to 350 degrees F. Grease a 9″-x-13″ baking pan and line with parchment paper.
- Whisk together flour, baking powder, and salt in a bowl. Stir together milk, lemon juice, and vanilla in a separate bowl.
- Beat butter and sugar with an electric mixer on medium speed until fluffy, 1 to 2 minutes. Add egg whites, one at a time, beating well after each addition. Reduce mixer speed to low and beat in flour mixture and milk mixture alternately, starting and ending with flour mixture, just until incorporated. Stir in sprinkles. Transfer batter to prepared pan; smooth top.
- Bake until a toothpick inserted in the center comes out clean, 40 to 50 minutes. Cool in pan on a wire rack, 10 minutes, then invert onto rack to cool completely.
- Transfer 3/4 cup frosting to a bowl. Add cocoa powder and stir to combine. Dye remaining frosting with green food coloring.
- Place template on cake and cut pieces with a long serrated knife according to the template.
- Assemble tree pieces on a serving platter, attaching them with a little green frosting, and frost green. Assemble stump, attaching pieces with a little brown frosting, and frost brown.
- Create star with yellow sour candy balls and mini M&M’s, garland with caramel popcorn, and decorate tree with red gumdrops, red sour candy balls, mini white and red M&M’s, regular white and red M&M’s, and starlight mints. Decorate stump with brown M&M’s and Raisinettes.