Preheat the oven to 350F (180C). Butter and dust with flour a 9 or 10 inch (23-25cm) bundt pan.
In a large bowl whisk flour with salt and baking powder. Set aside.
In another bowl mix butter with sugar and vegetable oil until creamy. Incorporate eggs one at a time. Add vanilla extract and sour cream and mix to combine.
With the mixer on low, gradually incorporate flour mixture until all is well incorporated.
Reserve 2 cups (500g) of vanilla batter. Set aside.
In a small bowl mix cocoa powder with sour cream and milk. Add to the reserved 2 cups of vanilla batter. Mix to incorporate.
Pour about up to half of the vanilla batter into the prepared pan. Carefully add the chocolate batter. Gently cover with the remaining vanilla batter.
Use a knife to create swirls.
Bake for about 45-50 minutes or until a toothpick inserted into the center comes out clean. If browns too fast, you can cover it with aluminum foil and continue baking.
Allow to cool slightly into the pan onto a rack for about 10-15 minutes. Invert and cool completely.
Dust with powdered sugar before serving or top with your favorite frosting.