Preheat the oven to 375°F. Generously grease a 12-cup muffin pan with butter or nonstick spray, skipping the liners so the muffins can be fully coated later.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, nutmeg and cinnamon.
In a large bowl, whisk the sugar and melted butter until combined. Add the buttermilk, sour cream and vanilla and whisk until smooth.
Add the dry ingredients to the wet ingredients and fold gently with a spatula until just combined. A few small lumps are fine, and stopping here keeps the muffins soft and cakey like a real donut.
Divide the batter evenly among the muffin cups, filling each about 3/4 full. Bake for 16-18 minutes, until the tops spring back when touched and a toothpick comes out clean.
Let the muffins cool in the pan for 5 minutes. Meanwhile, pour the melted butter into a small bowl and mix the sugar and cinnamon in another bowl.
While the muffins are still warm, dip each one into the melted butter, then roll it in the cinnamon sugar until completely coated. Serve warm or at room temperature.