In a large bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg.
Next, in a small bowl, combine the vinegar with the milk. Set aside for 5 minutes. This mixture creates buttermilk.
In a separate bowl, use a stand mixer or hand mixer to beat together the brown sugar with 2 tablespoons of melted and cooled butter (use vegan butter for dairy-free).
Once combined, mix in the egg, vanilla, and the buttermilk.
Add the wet ingredients to the dry ingredients and whisk together until just combined. Be sure to not over-mix.
Grease a 12-cavity medium donut pan with cooking spray.
Add the batter to a piping bag or a large Ziploc bag and cut off the corner. Pipe the batter into each donut cavity, filling about ½ to ⅔ full.
Bake at 350°F for 10-12 minutes, or until they are a pale, golden-brown color and a toothpick inserted in a donut comes out clean.
Allow to cool for at least 5 minutes.
While the donuts cool, melt ¼ cup of butter and allow to cool slightly.
Add ¼ cup of powdered sugar to a Ziploc bag. Working one or two donuts at a time, dip each side of the donut into the melted butter. Place in the Ziploc bag and seal. Then, shake the bag gently until the donuts are covered in the powdered sugar.
The powdered sugar does start to clump up because of the butter. When this happens throw some of that sugar away and add a fresh ¼ cup of sugar.
Repeat the coating process until done.
Notes
My top favorite gluten-free all purpose flours are: