Preheat oven to 400 degrees F.
Start by mixing together your dry ingredients in a bowl: flour, baking powder, cinnamon, and salt. Set this aside.
Then you're going to cream together room temperature butter and granulated sugar using a stand mixer. Beat this until fluffy - I usually do this for about two minutes.
Now mix in the egg, eggnog, and vanilla. One the egg, eggnog, and vanilla have fully incorporated, use a spatula (not the mixer) to stir in the dry ingredients. Be careful not to over mix! :)
Next scoop your batter into a prepared (sprayed with cooking spray) mini muffin tin. I use a cookie scoop, as I do with almost all of my "mini" or cookie recipes, to scoop the batter into the muffin tin.
Now bake for about 8-10 minutes. Remove from the oven and let rest in the muffin pan for about 10 minutes.
While the eggnog donut muffins are are cooling, melt 1/2 stick of butter in a small bowl and in another bowl place about 1 cup of powdered sugar. Remove the muffins from the pan and place onto a cooling rack. Taking one donut muffin at a time, fully submerge in the melted butter and then coat generously with powdered sugar. Place back on the cooling rack and serve immediately!