Prep Pan: Preheat oven to 325°F. Line a 9-inch springform pan with parchment and grease lightly.
Make Cake Base: Cream butter and sugar until fluffy. Beat in egg, then mix in cocoa, food coloring, vanilla. Add flour alternately with buttermilk. Stir in baking soda and vinegar. Spread evenly in pan.
Parbake: Bake base for 10 minutes. Remove from oven.
Make Cheesecake: Beat cream cheese and sugar until smooth. Add eggs one at a time, mixing well. Stir in vanilla and sour cream.
Assemble: Pour cheesecake layer over red velvet base. Tap to smooth.
Bake: Place pan in a roasting pan and fill with 1 inch of hot water. Bake 50–60 minutes until edges are set and center jiggles slightly.
Cool & Chill: Turn off oven, crack door, and cool 1 hour. Chill overnight.
Serve: Top with whipped cream and crumbs or chocolate shavings.