Preheat oven to 350°F (177°C). Spray mini muffin pan with cooking spray. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed until creamy and combined, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) On medium speed, beat in the egg and vanilla extract. Scrape down the sides and up the bottom of the bowl with a silicone spatula and beat again as needed to combine. (Mixture may appear curdled—that’s ok.) Add the flour, baking powder, cinnamon, and salt. Beat on low speed and with the mixer running, slowly pour in the milk and beat on low until combined. The batter will be thick.

Spoon batter into mini muffin wells filling 2/3 full.

Bake for 12-14 minutes or until a toothpick inserted in the center comes out clean or when lightly poked, the donut muffin bounces back. Remove from the oven and cool in the muffin pan for 5 minutes.

As the donut muffins cool, prepare the topping. Melt butter in a heatproof bowl in the microwave or melt over low heat on the stove. Pour the confectioners’ sugar in a medium bowl. Lightly dunk a muffin (they can still be warm at this point) in the melted butter. Submerge in the confectioners’ sugar to coat the surface generously. Set upright on cooling rack and repeat with remaining donut muffins.

Donut muffins taste best eaten on the same day. Cover leftover donuts and store at room temperature for up to 1 day or in the refrigerator for up to 1 week.