Place 1 stick unsalted butter in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Let sit at room temperature until softened. Meanwhile, coat a standard 12-well muffin pan generously with cooking spray, or line with paper liners.

Arrange a rack in the middle of the oven and heat the oven to 400ºF.

Make the topping: Place 3 tablespoons unsalted butter in a small microwave-safe bowl. Microwave on high until melted, about 30 seconds. Place ¼ cup granulated sugar, ½ teaspoon ground cinnamon, ⅛ teaspoon ground mace or nutmeg, and 1 pinch kosher salt in a small bowl and stir to combine.

Make the muffins: Place 2 cups all-purpose flour, 1 ¼ teaspoons kosher salt, 1 teaspoon baking powder, ½ teaspoon baking soda, and ½ teaspoon ground mace in a medium bowl and whisk until combined.

Add ¾ cup granulated sugar to the bowl of butter. Beat with the paddle attachment on high speed until pale colored and fluffy, about 2 minutes. Beat in 2 large eggs, one at a time, scraping down the sides of the bowl after each addition. Add 1 cup sour cream and 1 ½ teaspoons vanilla extract and beat until combined.

Add about ¾ of the flour mixture to the butter mixture. Fold in by hand with a rubber spatula until just combined. Add the remaining flour mixture and stir until just combined, do not overmix.

Divide the batter evenly between the muffin wells. Brush the top of each muffin generously with the melted butter. Sprinkle with the sugar-cinnamon mixture.

Bake for 8 minutes. Do not open the oven door and reduce the oven temperature to 350°F. Bake until a toothpick inserted into the center of a muffin comes out clean, 8 to 10 minutes more. Let the muffins cool in the pan for 5 minutes. Remove the muffins from the pan and serve warm or transfer to a wire rack to cool completely.