Preheat oven to 325°F. Grease and line a loaf pan with parchment.
In a bowl, whisk together all dry ingredients except the cocoa powder.
1 & 1/2 cup all-purpose flour, 1 cup granulated sugar, 1/2 teaspoons baking soda, 1/4 teaspoons baking powder, 1/2 teaspoons fine salt
In another bowl, whisk melted butter, oil, sour cream, milk, vanilla, eggs, and egg yolk. Mix until smooth.
6 tbsp butter, 1/4 cup vegetable oil, 1/2 cup full-fat sour cream, 2 large eggs, 1 egg yolk, 1.5 teaspoons pure vanilla extract, 2 tbsp whole milk
Add dry ingredients to wet and fold just until combined.
Divide batter evenly into two bowls. Mix cocoa powder, red food coloring, and vinegar into one of the cake batter bowls, and mix until combined
1 tablespoon unsweetened cocoa powder, 1/2 tablespoon red gel food coloring, 1/2 teaspoon distilled white vinegar
Spoon alternating batters into the pan. Gently swirl with a knife (2–3 times only).
Bake for about 50 minutes, tent with foil if needed. Check doneness with a toothpick (a few moist crumbs is perfect).
Cool in pan for 20 minutes, then transfer to cool completely.