Cinnamon Streusel Topping:

1. In a small mixing bowl, whisk together the flour, granulated sugar, brown sugar, cinnamon, and salt. Add the melted butter and mix in until all of the dry ingredients are evenly moistened and the mixture is clumpy. Place in the refrigerator to chill while prepping the muffins.

Cinnamon Swirl:

1. In a small bowl, whisk together the granulated sugar, brown sugar, and ground cinnamon. Set aside.

Muffin Batter:

1. Preheat oven to 400ºF and grease a standard 12-cup muffin pan with nonstick cooking spray.
2. In a medium mixing bowl, whisk together the flour, baking powder, salt, and baking soda.
3. In the bowl of a stand mixer fitted with the paddle attachment, or using a large bowl and handheld electric mixer, cream together the butter and sugar on medium-high speed until light and fluffy.
4. Reduce mixer speed to medium. Beat in the eggs one at a time until smoothly incorporated, wiping down the bowl and beater as needed. Beat in the sour cream and vanilla extract.
5. Add a third of the flour mixture to the butter and sugar mixture, mixing in on low speed until mostly incorporated. Continuing on low speed, mix in half of the buttermilk, the second third of the flour mixture, and finally the remaining half of the buttermilk, mixing each addition in until mostly combined. Finish folding the batter together with a spatula, just until everything is evenly combined. The batter will be fairly thick.

Assembling and Baking:

1. Fill the prepared muffin cups about halfway with batter, gently pressing the batter down into each cup to ensure the bottoms are completely covered. Divide the cinnamon-swirl mixture evenly over the muffin batter in each cup.
2. Top with the remaining batter to cover the cinnamon completely; the muffin cups will be full. Use a toothpick to swirl the batter in each cup, and then gently smooth out the tops of the muffins with your fingertip.
3. Sprinkle the streusel mixture over the muffins, breaking up any especially large clumps with your fingers in the process.
4. Bake muffins in the preheated oven for 7 minutes. Reduce heat to 350ºF and bake for another 15-18 minutes - until a toothpick inserted into the center of one comes out clean or with just a few moist crumbs. Place pan over a wire cooling rack and allow muffins to cool to room temperature before removing.