Beat butter at medium speed with an electric mixer until creamy. Gradually add sugar, beating until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition.
Stir together flour, cocoa, and baking soda. Add to butter mixture alternately with sour cream, beating at low speed just until blended, beginning and ending with flour mixture. Stir in vanilla; stir in red food coloring. Use batter immediately, following baking directions for desired cake in "Pick a Pan" below.
Pick a Pan: Chocolate-Red Velvet Cake Batter can be baked in lots of different shapes and sizes--just use these times as a guideline, and be sure to grease and flour your pans. With smaller muffin pans and molds, we found it easier to use a vegetable cooking spray with flour. Check for doneness at the minimum time range, continuing to bake until a wooden pick inserted in the center comes out clean. Variations with added fruit and nuts will require the maximum time and yield more mini cakes.
Bake at 325° in 1 (12-cup) tube pan for 1 hour and 15 minutes to 1 hour and 30 minutes.
Bake at 325° in 3 (8- x 4-inch) loaf pans for 55 to 65 minutes.
Bake at 325° in 7 to 9 (5- x 3-inch) loaf pans for 25 to 35 minutes.
Bake at 325° in baby Bundt pans for 25 to 30 minutes. Makes 14 to 16 baby Bundt cakes. (Spoon 1/2 cup batter in each 1-cup mold.)
Bake at 350° in muffin pans 23 to 28 minutes. Makes 32 to 38 cupcakes.
Bake at 350° in miniature muffin pans for 9 to 11 minutes. Makes about 9 to 10 dozen cupcakes.