In a large mixing bowl whisk the flour, baking powder, baking soda, cinnamon, and salt. Make a well in the center once all the ingredients are evenly mixed.
Using an electric mixer or stand mixer cream the eggs, sugar, and vanilla together until it turns pale and thick.
Slow pour in the melted butter and beat until combined. Add half the sour cream in and beat to just mixed, and then add the remaining sour cream and repeat.
Pour the wet ingredients into the well of the dry ingredients and gently fold in until fully mixed using a spatula.
Use the spatula to turn the dough out onto plastic wrap and wrap it up tightly. Place in the refrigerator to chill for a minimum of one hour and up to overnight.
Using a heavy-bottomed pot, such as a Dutch oven, add 2-inches of oil and place over medium heat. Clip a candy thermometer to the side of the pan or measure the heat using another type of food thermometer. Bring the oil to exactly 350ºF.
Roll the dough out to 1/2-inch thickness on a floured surface. Using a donut cutter, if you have one, or a biscuit cutter, cut out 12 donuts. Don't pierce the dough all the way to the bottom, stop about halfway. Then gently pull the excess dough off.
Once the oil is heated to 350ºF place two to three donuts at a time in the oil to cook. The donuts will float to the top and when they do allow them to cook 1 more minute before flipping them over to cook for an additional 1 minute. The donuts should be a nice golden brown.
Use a slotted spoon to carefully remove the donuts from the oil and place on paper towels to cool (not a wire rack), which takes about 5 to 10 minutes.
In a medium-sized bowl whisk the powdered sugar, vanilla, and milk together until smooth. Once the donuts have cooled dip them into the glaze on both sides.