Prepare Pans: Grease two round cake pans and line with parchment paper.
Mix Dry Ingredients: In a bowl, whisk flour, cornstarch, cocoa powder, baking soda, baking powder, and salt.
Cream Butter & Sugar: Beat butter, oil, and sugar until light and fluffy.
Add Eggs & Wet Ingredients: Add eggs one at a time, then add vanilla, vinegar, and red food coloring. Mix well.
Combine Wet & Dry: Add half of dry ingredients, mix gently, add buttermilk, then fold in remaining dry ingredients. Do not overmix.
Bake: Divide batter into pans. Bake until a toothpick comes out clean. Cool completely before frosting.
Make Frosting: Beat butter until creamy. Gradually add powdered sugar. Add cream cheese, vanilla, and lemon juice. Beat until smooth.
Assemble Cake: Frost between layers and all around the cake. Decorate with rosettes if desired.
Serve & Enjoy: Slice and enjoy your soft, moist red velvet cake!