Preheat the oven to 315°F (yes, that is correct!)
Prepare a 9 or 10-inch Bundt pan with nonstick cooking spray and generously dust with flour, tapping excess flour out. NOTE: I prefer to spray my cake pans with a nonstick spray such as Baker's Joy so that I don't need to add flour. In a medium bowl, whisk together the flour, baking soda, and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment (or with a hand mixer), combine the sugar and butter. Beat on medium speed until pale and fluffy, about 3 minutes.
Add the eggs, one at a time, beating well after each addition until the mixture comes together as creamy.
On low speed, add the sour cream and vanilla. Continue mixing until the batter is looks creamy, scraping down the sides and bottom of the bowl. You'll want to make sure all of the ingredients are evenly incorporated. Gradually add the flour mixture on low speed until the batter is smooth with no lumps or bumps.
Scoop about 1/3 of the batter into the prepared pan. NOTE: I find that using an ice cream scoop makes this less messy, and helps to evenly distribute the batter.
Sprinkle half of the cinnamon mixture evenly over the batter.
Evenly distribute another 1/3 of the remaining batter. (TIP: An off set spatula, helps to gently spread the batter.)
Sprinkle the remaining cinnamon mixture evenly over the batter and cover with the remaining batter, spreading it evenly over the cinnamon mixture.
Bake between 65-75 minutes, depending on your oven, until the cake is golden and a cake tester comes out clean. Cool the cake in the pan for 15 minutes-- but no longer than that, lest the cinnamon mixture might stick to the pan. Place a wire rack on top of the Bundt pan and then invert the pan to release the cake.
Let the cake cool completely before serving (though, I admit I cut into it while still warm).
Store in an air-tight container.