In a medium bowl, whisk together 1 1/4 cup all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cinnamon and 1/4 teaspoon kosher salt. Set aside.
In a separate medium bowl, whisk together 1/3 cup granulated sugar, 1/4 cup sour cream, 1 large egg and 1 tablespoon unsalted butter until smooth. Add the flour mixture (from the 1st step) a little at a time until a smooth dough forms. Cover the bowl with plastic wrap and refrigerate the batter for 15 to 20 minutes.
After the batter has chilled, heat at least 2 inches of oil in a deep fryer or a cast iron skillet until a candy thermometer registers 360 (F).
Roll out the dough on a lightly floured surface to 1/2 inch thick. Use a 3 1/2 inch donut cutter to cut out donut holes.
With a metal spatula, carefully place 1 or 2 donuts at a time in the oil. Fry for 1 to 2 minutes per side, or until light golden brown. Remove with a slotted spoon and drain on a paper towel on a wire rack. Let cool slightly before glazing. Like most quick breads, the donuts are best on the day they are made.