1. Preheat oven to 325ºF. Spray a 9-inch Bundt pan with non-stick spray, coat with flour, tapping out excess and set aside.
2 .Combine the brown sugar and cinnamon together in a small bowl and set aside. In a separate bowl, combine the flour, baking soda and salt, whisking together. Set that aside too.
3. Place the sugar and butter in the large bowl of a mixer and beat on medium speed until creamy and light. Add in the eggs, one at a time, beating well after each one and using a rubber spatula to scrape down the sides and bottom of the bowl as needed. Add in the sour cream and vanilla and mix just until incorporated. Scrape down the sides of the bowl well and gradually add the flour mixture, keeping the mixer on low speed. (You will need to scrape the sides down a lot as the mixture is very thick and you want to make sure it gets evenly mixed.)
4. Spoon about 1/3 of batter into prepared bundt pan and smooth it down evenly. Sprinkle 1/2 of the cinnamon mixture evenly across it and top that with another 1/3 of the batter, spreading it out evenly to the sides. Sprinkle the rest of the cinnamon mixture evenly across the top and follow it with the rest of the batter, spreading it out evenly. Use the top of a sharp knife to gently swirl the layers together just a bit.
6. Bake for about 60 minutes, until a cake tester inserted into the center comes out clean (it might take longer/shorter depending on your oven and pan. Mine needed 55 minutes ). Let the cake cool for about 15-20 minutes in the pan on a wire rack and then invert and remove cake. Let cool before slicing and serving. (I enjoyed it a bit warm!)
7. Cake keeps well wrapped at room temperature for up to a week.
Recipe adapted slightly from: Zingerman's Bakehouse Cookbook
*I reduced the sugar by 1/4 cup to account for extra brown sugar in the filling.