Preheat the oven to 325 degrees.

Grease and flour two, 8 inch round cake pans and set aside.

In a bowl, combine the flour, baking soda, baking powder and salt. Whisk well and set aside.

In another bowl, add the oil, sour cream, milk, and vanilla extract. Whisk well and set aside.

In a large mixing bowl, add the room temperature butter and the sugar and mix on medium and then on medium high for several minutes until light and fluffy.

Add in the eggs one at a time and mix on medium just until combined after each. Only mix until the egg is just incorporated. Try not to over mix. It's best if your eggs are room temperature, but if they're cold, your batter may look a little curdled at this point.

Scrape down the sides of the bowl and mix again on medium, just until everything is incorporated.

Alternately add in the flour mixture and the liquid mixture. Starting and ending with the flour mixture. (Add in ⅓ of the flour mixture and mix on medium just until combined. Add in ½ the liquid mixture, mixing only until combined. Add in another ⅓ of the flour mixture, mixing only until combined. Add in the last of the liquid mixture, mixing only until combined, then the last of the flour mixture and mix until combined.)

Scrape down the sides of the bowl and mix again only for 5-7 seconds or so. You only want to mix until well combined. Mixing for several minutes is too long for a scratch cake. The batter will be thick.

Add about 1 to 1 ½ cups of the batter into a small mixing bowl. (If you want less chocolate swirl, use 1 cup, if you want more chocolate swirl use 1 ½ cups)

To that batter, add the cocoa powder, 1 teaspoon oil and the 3 tablespoons of milk and mix well with a spoon or rubber spatula. If the batter is very stiff, mix in another tablespoon of milk.

Add the chocolate batter back into the vanilla batter by tablespoonfuls. Then swirl the batter in the bowl with a spoon or rubber spatula. Just marble the batter, don't mix it completely.

Spread into two prepared 8" round cake pans. (The batter will be thick.)

Bake at 325 degrees for approximately 40-45 minutes. Insert a toothpick somewhere in the middle and a few moist crumbs should stick to the toothpick.

Cool on a wire rack, in the pans, for about 10 minutes, then turn the cakes out onto the rack and allow them to cool completely before freezing the layers, or covering in buttercream.