Preheat your oven to 350 degrees, and generously grease a 9 inch loaf pan. Set aside.
In a medium bowl, whisk together the flour, baking powder, and salt and set aside. In a large bowl, beat together the butter and sugar until fluffy. Beat in the eggs one at a time until fully combined. Beat in the milk and vanilla extract. Slowly mix in the flour mixture until fully combined and smooth.
Scoop roughly 1/3 of the batter into another bowl (no need to measure this) and add the cocoa powder, red food coloring, and white vinegar to this mixture. Stir together until it is completely combined.
Scoop both batters in alternating blobs into your prepared cake pan until all of the batter has been added. Using a butter knife or tall skewer, swirl the cake batter using about 4-6 figure-eight motions until you see a swirled effect.
Bake in your preheated oven for 1 hour and 10 minutes, or until a cake tester comes out clean. Cool the cake completely before removing from the pan to slice and serve!