1Combine the sugar and cinnamon in a small bowl and set aside.
For the muffins:
1Heat the oven to 375°F and arrange a rack in the middle. Lightly brush a 24-well mini muffin pan with some of the melted butter, then coat it with flour, tapping out any excess; set aside. Reserve the remaining melted butter for applying the cinnamon-sugar coating.
2In a large bowl, sift together the measured flour, baking powder, salt, nutmeg, and baking soda. In a small bowl, whisk together the milk and buttermilk. Set both aside.
3In the bowl of a stand mixer fitted with a paddle attachment, beat the room-temperature butter and sugar on medium-high speed until light in color and fluffy, about 3 minutes. Beat in the eggs 1 at a time until combined. Stop the mixer and scrape down the sides of the bowl with a rubber spatula.
4With the mixer on low speed, beat in a quarter of the reserved dry ingredients. Then beat in a third of the reserved milk mixture. Continue to alternate until all of the remaining ingredients are incorporated, finishing with the dry ingredients. Do not overmix.
5Fill the prepared muffin pan wells just to the rim with batter. Bake until the muffins are lightly golden and firm to the touch, about 15 to 20 minutes. Let cool in the pan for 5 minutes. Transfer the muffins to a wire rack set over a baking sheet. Repeat the buttering and flouring of the muffin pan and bake the remaining batter.
6To coat the muffins, brush each generously with the melted butter and sprinkle generously with the cinnamon-sugar mixture. Serve warm or at room temperature.
Variation: Jelly Doughnut Muffins
If jelly doughnuts are more your style, allow the muffins to cool and use a small pastry tip to make a hole in the bottom of each. Fill the pastry bag with your favorite jam, jelly, or citrus curd and squeeze some into the muffins.