Preheat the oven to 350°F/175°C. Line three 8" pans with parchment rounds and grease with nonstick baking spray or homemade cake release. Whisk together 3 cups flour, 3 cups granulated sugar, 2 1/2 tsp baking powder, 2 tsp ground cinnamon, and 1 tsp fine salt in a large bowl or the bowl of a stand mixer.
Mix 1/2 cup of room-temperature butter slowly into the dry ingredients on a low speed. Continue to mix until no large chunks remain.
In a separate bowl, whisk together 4 large eggs, 1 1/2 cups sour cream, 1/2 cup vegetable or canola oil, and 1 tsp of vanilla bean paste or extract.
Slowly mix the wet ingredients into the dry ingredients on a low speed until just incorporated. Scrape down the sides of the bowl with a rubber spatula, then beat on a medium speed for 1 more minute to make sure everything is properly mixed.
Add about 1 cup (250g) of cake batter to each of the prepared pans and spread into an even layer. Sprinkle 1/4 cup of the cinnamon swirl mixture over each pan.
Divide the remaining cake batter evenly between the three pans and drizzle it over the cinnamon sugar mixture (about 350g per pan). Carefully spread the batter over the cinnamon swirl mixture with a small offset spatula.
Bake for 33-36 minutes or until a toothpick comes out with a few moist crumbs. Let the cake layers cool in the pans for 30 minutes, then run a small offset spatula around the perimeter of the pan to separate the cake from the pan. Carefully flip the cake layers onto a wire rack once they're fully cool.
Use a serrated knife to level the tops of the layers once they're fully cooled. If you make these cake layers in advance and freeze them, let them thaw for about 20 minutes before making your cake. The cake layers should still be slightly cold to the touch, which will make it easier to assemble your cake.