Preheat the oven to 350°F and generously coat a 12-cup muffin pan with nonstick spray.
cooking spray
In a bowl, whisk the flour, baking powder, salt, baking soda, and nutmeg together.
2-1/2 cups all-purpose flour, 1-1/2 tsps. baking powder, 1/2 tsp. table salt, 1/4 tsp. baking soda, 1/4 tsp. ground nutmeg
In a large bowl, blend the buttermilk, milk, sweet potato puree, and vanilla until smooth.
1/3 cup buttermilk, 1/3 cup whole milk, 1/2 cup sweet potato puree (from a baked sweet potato), 1 tsp. vanilla extract
Using an electric mixer (handheld or stand), cream the butter and brown sugar together until fluffy. Add the eggs one at a time, beating well after each addition. Gradually alternate adding the dry and wet mixtures to the butter mixture, beginning and ending with the dry ingredients—take care not to overmix.
1 stick (8 tbsps.) unsalted butter, room temperature, 3/4 cup light brown sugar, 2 large eggs
Fill each muffin cup halfway, and bake for about 25 minutes, or until a toothpick inserted into the center comes out clean.
For the coating, combine the granulated sugar, cinnamon, and nutmeg in a shallow dish. When the muffins are warm but easy to handle, remove them from the pan, brush with melted butter, and roll each in the sugar mixture.
1/2 cup granulated sugar, 2 tsps. ground cinnamon, 1/4 tsp. ground nutmeg, 1 stick (8 tbsps.) unsalted butter, melted