Cinnamon Buttermilk Muffins

7 tbsp. unsalted butter, at room temperature
2/3 c. sugar
1 egg
1 1/2 c. all-purpose flour
1 1/2 tsp. baking powder
1/2 tsp. baking soda
1/2 tsp. salt
1/2 tsp. freshly grated nutmeg
1/2 c. buttermilk
1 1/2 tsp. vanilla extract

Topping:
2/3 c. sugar
1 tbsp. ground cinnamon
6 tbsp. (3/4 stick) unsalted butter, melted

Preheat oven to 350°F. Grease 9 standard muffin cups with butter or butter-flavored nonstick cooking spray; fill unused cups 1/3 full with water to prevent warping.

To make muffins, in bowl of stand mixer fitted with flat beater, combine butter and sugar and beat on medium speed until light and fluffy. Add egg and beat well until pale and smooth. In another bowl, stir together flour, baking powder, baking soda, salt, and nutmeg. Add to butter mixture in 2 additions, alternating with buttermilk and vanilla. Stir just until evenly moistened. Batter will be slightly lumpy. Spoon batter into prepared muffin cups, filling each 3/4 full. Bake until muffins are golden, dry, and springy to the touch and a toothpick inserted into center of muffin comes out clean, 20 to 25 minutes. Transfer pan to wire rack and let cool for 5 minutes. Unmold muffins and let stand until cool enough to handle.

To make topping, in a small, shallow bowl, stir together sugar and cinnamon. Put melted butter in another small bowl. Holding bottom of muffin, dip top into melted butter, turning to coat evenly. Immediately dip top in cinnamon-sugar mixture, coating evenly, then tap to remove excess sugar. Transfer muffin, right side up, to rack. Repeat with remaining muffins. Let cool completely before serving. Makes 9 muffins.