Preheat oven to 350°F. Line 9x13 pan with parchment paper.
Mix dry ingredients: flour, cocoa, baking soda, salt.
Beat butter and sugar until combined.
Add eggs one at a time, then red food coloring, vanilla, vinegar.
Alternate adding dry ingredients and buttermilk to wet mixture.
For frosting: Beat cream cheese, sugar, egg, and vanilla until smooth.
Pour batter into pan, add cream cheese mixture in dollops.
Swirl with knife to create marbled effect.
Bake 30-35 minutes until toothpick comes out with few crumbs.
Cool completely before cutting.