Preheat the oven to 350F degrees and spray a mini doughnut pan with non-stick cooking spray. You'll end up with about 30-36 mini doughnuts total, so you'll either need 3 pans or to bake them in batches.

In a large bowl whisk together the flour, nutmeg, baking soda & salt & set aside.

In a separate medium sized bowl whisk together the melted butter, sugars, egg & vanilla. Once combined and no sugar lumps remain whisk in the sour cream & buttermilk. Pour the wet ingredients into the dry and whisk until combined.

Spoon the batter into a large ziploc bag, then seal the bag and cut one of the bottom corners off. Pipe the batter into the doughnut pan, filling each doughnut cavity about ½ full. You should end up with about 30-36 mini doughnuts total.

Bake for 5-7 minutes, or until when you gently touch the top of the doughnut it springs back and feels slightly firm. Remove from the oven and allow to cool in the pan for 5-10 minutes, then remove the doughnuts from the pan and continue cooling on a wire rack.

Once the doughnuts have cooled, in a small bowl stir together the sugar and cinnamon for the topping. Then setup your work space so that you have a small bowl with your melted butter, the bowl with your cinnamon sugar, a wire cooling rack (with paper towels underneath), and your pastry brush(if using). For each doughnut, brush both sides with melted butter shaking off any excess, then roll each side in the cinnamon sugar and place on the wire rack. Repeat until you've finished all of the doughnuts. If you don't have a pastry brush, you can dip each side of the doughnut in the melted butter and then shake off any excess.