Streusel:

Melt butter in a medium, microwave-safe bowl.

Add sugars, cinnamon, and salt and whisk until incorporated.

Fold in flour until combined and use hands to clump into a ball. Set aside while prepping remaining components.

Cinnamon-Swirl:

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

Muffins:

Preheat oven to 400ºF and grease a 12-cup muffin tin with oil or nonstick spray.

In a medium bowl, whisk together the flour, baking powder, salt, and baking soda.

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 until light and fluffy.

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.

Beginning and ending with the dry ingredients, add 1/3 of the dry mixture to the butter/sugar mixture, alternating with 1/2 of the buttermilk, mixing on low speed and continuing until all parts have been added and are mostly incorporated. Finish folding together by hand just until evenly combined.

Fill prepared muffin cups about halfway with batter, making sure to cover the bottom of each tin completely. Divide cinnamon-swirl mixture evenly between each muffin cup, and top with remaining batter to cover cinnamon completely. Swirl batter with a toothpick as best as possible (it will be thick). Smooth tops of muffins with your fingertip.

Use hands to break streusel mixture into clumps and scatter over prepared muffins.

Bake muffins in preheat oven for 7 minutes. Reduce heat to 350ºF and bake another 13-15 minutes, until a toothpick inserted into the center comes out clean or with just a few moist crumbs.

Allow muffins to cool to room temperature before removing from pan. Muffins are best served fresh.