Preheat the oven to 350 degrees Fahrenheit, and generously butter a 9×5-inch loaf pan.

In a measuring cup add the milk and vinegar. Set aside for 10 minutes to “sour".

In a bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

In a separate large bowl, use a hand mixer or stand mixer fitted with a paddle or whisk attachment to beat the softened butter and sugar together on medium-high speed for 2 minutes or until smooth and creamy.

Add the eggs, and beat on high speed for 1 minute. Then, beat in the sour cream and vanilla extract, scraping down the sides and bottom of the bowl as needed. Note: The mixture may appear lumpy, but that’s normal!

Gradually pour the dry ingredients into the bowl of wet ingredients. Mix on low speed just until combined. Be careful not to overmix!

Transfer 1 cup of the prepared batter to a separate, clean bowl. Stir in the vegetable oil and cocoa powder unitl smooth.

Layer and swirl the batters: Spread a thin layer of the plain vanilla batter in the bottom of the prepared loaf pan. Alternate spoonsful of hte chocolate batter and vanilla batter on top, creating a layered effect until all of the batter has been used. Use a knife or wooden skwere to swirl the batters around, creating a marbled appearance.

Transfer the pan to the oven, and bake for 65-75 minutes or until a toothpick can be inserted into the center and comes out clean. NOTE: If the top of the cake begins to brown too quickly, loosely cover it with aluminun foil, and continue to bake.

Allow the cake to cool completely before inverting into a plate and serving.

Store at room temperature for 2-3 days or in the fridge for up to 5 days.