Preheat the oven to 350 degrees F, grease and flour a bundt pan. We used a bundt pan with a 12 cup baking capacity, 10 inches across. See notes re: pan size
In a medium sized bowl add the cake flour, baking powder, baking soda and salt. Whisk to blend, set aside.
In another bowl or measuring cup add the sour cream, milk, vegetable oil, and vanilla extract. Blend with a fork and set aside
In the bowl of your mixer, mix the softened butter until smooth. Gradually add the sugar and mix on medium speed for 3 to 5 minutes until it is lightened in color and fluffy.
Add the eggs one at a time, mixing after each until the yellow of the yolk has blended.
With the mixer on low speed, add the flour mixture and the sour cream mixture alternately, beginning and ending with the flour mixture (We did 3 additions of dry ingredients and 2 of wet). Mix just until combined.
Add 3.5 cups(about half) of the cake batter to the prepared bundt pan.
Then, sprinkle the cinnamon and brown sugar mixture over the batter. We recommend not carrying the mixture all the way to the edge of the pan or to the center tube/core of the pan. (We explain this in the notes.)
Top with the remaining cake batter and gently smooth with the back of a spoon. Tap the filled cake pan on your counter a few times to make the air bubbles rise.
Bake at 350 degrees for 45-50 minutes and then check for doneness-it may need additional time. Bake until a toothpick inserted into the center comes out clean or with just a few crumbs attached. Allow the cake to cool in the pan, on a wire rack for 10 minutes before turning out.
Makes 7 cups of cake batter