• Preheat your oven to 350°F. Grease a standard-size donut pan and set it aside.
• In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, kosher salt, nutmeg, and sugar, and whisk to combine well.
• Create a well in the center of the dry ingredients, add the butter, eggs, yogurt, and vanilla, and mix until just combined.
• Fill the prepared wells of the donut pan until nearly full. For perfectly shaped donuts, place the donut batter into a piping bag fitted with a large, open tip, and pipe the batter into the wells.
• Wet the pointer finger of your dominant hand and run it along the center of each well of the donut pan to force the batter toward the perimeter of each well. This will help the donut hole from closing during baking.
• Place in the center of the preheated oven and bake for about 12 minutes or until the donuts are set and just lightly browned.
• Remove the pan from the oven, and allow the donuts to cool in the pan for 5 minutes.

For the optional glaze.

• Transfer the donuts to a wire rack to cool completely.
• In a small bowl, place the confectioners’ sugar and 2 teaspoons of milk, and mix well. Add more milk by the half-teaspoon until you have a smooth but thickly pourable glaze.
• Working quickly, dip the top of each cooled donut in the glaze, turn back and forth a bit to coat well, invert the donut so the glaze is facing upward, and place on a flat surface to set.

For the optional sugar-coating.

• Invert the donuts onto a wire rack while they’re still warm.
• Dip each donut carefully in the melted butter and then press into the granulated sugar before replacing on the wire rack. The sugar coating will harden as it cools.