Preheat the oven to 350° and grease your donut pan.
Stir together all the dry ingredients (including any optional add-ins listed above) in a large bowl. Add in egg, milk (or eggnog if making eggnog donuts), vanilla, avocado oil, and plain Greek yogurt. Hand mix until just combined, don't over mix your batter.
Scoop or spoon your batter into the donut pan. Fill each donut mold ¾ full of batter, for about 8 donuts.
Bake at 350° for 14-15 minutes, until cooked through and toothpick comes out clean. Allow donuts to cool for a few minutes, then transfer to a cooling rack.
While the donuts are cooling, make the glaze.
Gingerbread glaze: whisk together 1 cup powdered sugar, 1 tbsp whole milk, and ½ tsp vanilla extract until smooth. Eggnog glaze: whisk together 1 cup powdered sugar, 1 tbsp eggnog, and ½ tsp vanilla extract until smooth. Hot cocoa glaze: whisk together 1 cup powdered sugar, 2 tbsp whole milk, ½ tsp vanilla, and ¼ cup hot cocoa mix until smooth.
Once the donuts are cool enough to handle, dip the donuts in the glaze so the top half of each donut is covered in glaze. Place on a cooling rack, sprinkle hot cocoa donuts with mini or dehydrated marshmallows, sprinkle the gingerbread donuts with cinnamon sugar, and sprinkle the eggnog donuts with nutmeg.
Allow the glaze to harden for about an hour. Serve!
Notes
These donuts can be stored at room temperature for up to 3 days. I wouldn't recommend putting these in the fridge because they dry out pretty quickly.