Preheat oven to 350°F (180°C). Grease and flour a 10-cup bundt pan, two 8.5”x4.5” loaf pans, or two 9” round cake pans.
Combine the cocoa powder, ¼ cup sugar, red food coloring, and vinegar in a small bowl for the red velvet layer. Set aside.
Whisk the flour, baking powder, and salt in a bowl, and set aside.
Cream the softened butter and 1½ cups sugar until light and fluffy (3-5 minutes). Add vanilla, then beat in eggs one at a time.
Mix in sour cream until smooth and fully incorporated.
On low speed, mix in the flour in three additions, alternating with milk, starting and ending with flour. Do not overmix.
Pour ⅔ of the batter into your pan(s). Mix the chocolate red velvet mixture into the remaining ⅓ batter. Add more food coloring if needed for the right color. Spoon it on top of the vanilla batter and swirl with a knife or skewer.
Bake for 50-60 minutes in a bundt, 60-70 minutes for a loaf pan, or 30-35 minutes in round cake pans or until done (see notes).
Cool 10-15 minutes in the pan, then gently flip it over on a cooling rack, and allow to finish cooling. Dust with powdered sugar or drizzle with vanilla glaze.