muffins
Nonstick vegetable oil spray
2¼ cups all-purpose flour
2 teaspoons baking powder
1 teaspoon kosher salt
½ teaspoon ground nutmeg
½ cup (1 stick) unsalted butter, room temperature
¾ cup sugar
1 large egg
1 cup whole milk
topping and assembly
½ cup sugar
1 teaspoon ground cinnamon
6 tablespoons (¾ stick) unsalted butter, melted
Muffins
Preheat oven to 375°. Coat a standard 12-cup muffin pan with nonstick spray. Whisk flour, baking powder, salt, and nutmeg in a medium bowl; set aside.
Using an electric mixer, beat butter and sugar in a large bowl until light and fluffy, about 4 minutes. Beat in egg. With mixer on low speed, add dry ingredients in 3 additions alternating with milk in 2 additions, beginning and ending with dry ingredients.
Divide batter among muffin cups and bake, rotating pan halfway through, until a tester inserted into center comes out clean, 30–35 minutes. Let cool 5 minutes in pan, then transfer to a wire rack.
Topping and Assembly
Mix sugar and cinnamon in a medium bowl. Working one at a time, dip tops of muffins in melted butter, then cinnamon sugar.
Do Ahead: Muffins can be made 6 hours ahead. Keep tightly wrapped at room temperature.