Preheat the oven to 350°F. Spray a donut pan with non-stick spray. Set aside.
In a medium bowl, whisk together the all-purpose and cake flours, baking powder, baking soda, salt, and nutmeg. Set aside.
1 cup all-purpose flour, 1 cup cake flour, 2 teaspoons baking powder, ½ teaspoon baking soda, 1 teaspoon salt, ⅛ teaspoon ground nutmeg
In a separate medium bowl, whisk together the eggs, brown sugar, milk, and sour cream until smooth.
2 large eggs, ⅔ cup dark brown sugar, ½ cup whole milk, ½ cup full-fat sour cream
Add in the melted butter and vanilla, whisking until fully combined.
4 tablespoons unsalted butter, 2 teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not over mix. The batter will be thick.
Add the batter to a ziplock bag with ½ an inch of one corner cut off. Pipe the batter into the donut cavities, filling each about ⅔ of the way. Do not overfill.
Bake for 10-12 minutes or until the edges are just lightly browned.
Allow to cool for about two minutes and turn out onto a wire rack set on a large piece of parchment paper or over a baking sheet.
Allow donuts to cool down a couple of minutes until you can handle them. See notes for topping options