Preheat oven to 350 degrees. Butter and flour a large 10-15 cup bundt pan, set aside.

In the bowl of a stand mixer or a large mixing bowl with a hand mixer, beat butter until creamy, about 1 minute. Add sugar and beat on medium speed for 3 minutes or until pale. Add eggs one at a time, mixing after each addition. Add vanilla extract and mix until combined.

In a medium mixing bowl whisk together the flour, baking powder, baking soda, and salt. With mixing speed on low, alternate adding the dry ingredients and the milk and sour cream. Mix just until combined.

Remove 2 1/2 cups of batter and place in a small mixing bowl. In a small soup or cereal bowl, whisk together the cocoa powder, oil and water. Gently fold into the reserved batter until no streaks remain.

Using a standard size cookie scoop or a spoon, alternate adding yellow and chocolate batter to the pan. After the bottom of the pan is no longer visible, gently swirl the batter together (be careful not to overdo it). Repeat with two additional layers. Tap the pan on the counter to even the batter out.

Place in the oven and bake for 45-55 minutes or until a cake tester inserted into the center of the cake comes out clean, or the cake springs back when touched.

Remove from the oven and allow to cool for 15 minutes in the pan before inverting onto a wire rack to cool completely. When cool dust with powdered sugar if desired.