Glazed Sour Cream Doughnuts

First make the doughnuts by sifting together the flour, baking soda, cinnamon and salt into a bowl. Set aside. In a separate bowl, whisk together the sugar, sour cream, egg and butter. Add the flour mixture in batches to the sour cream mixture, folding it in with a spatula until fully incorporated into a smooth dough. Cover the bowl with plastic wrap and refrigerate the batter for 20 minutes. Roll the dough out onto a lightly floured surface until it is approximately 1/2-inch thick. Use a cookie cutter to cut out doughnuts that are 2 1/2 inches in diameter. Re-roll scrap dough until all doughnuts have been cut out. Optional: Reserve the centers of each doughnut to make doughnut holes. Prior to frying the doughnuts, prepare the sugar glaze by combining the sifted sugar, whole milk and vanilla extract in a small bowl. When ready to fry doughnuts, heat at least 2 inches of vegetable oil in a heavy-bottomed pot until it reaches 360 ºF. Fry the doughnuts and doughnut holes in batches until they are golden brown. Using a slotted spoon or spider, transfer the doughnuts to a cooling rack to drain. Let them cool just slightly before dipping them in the prepared glaze.