Preheat oven to 350F. Spray two 12-count mini muffin pans with floured cooking spray; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, combine egg, butter, sugars, vanilla, and beat on medium-high speed until light and fluffy, about 3 minutes.

Stop, scrape down the sides of the bowl, and add the buttermilk. Beat briefly to incorporate.

Add the remaining dry ingredients and mix on low speed until just combined; batter will be fairly thick.

Using a spoon sprayed with cooking spray, drop rounded tablespoons into prepared pans, filling each about 2/3 to 3/4 full.

Bake for 10 to 15 minutes until muffins are domed, set, springy, and a toothpick comes out clean. Cool for 10 minutes before removing.

For the topping, melt butter in the microwave, stir together sugar and cinnamon, then dip muffins in melted butter followed by cinnamon sugar.