Preheat oven to 375°F. Whisk together oat flour, granulated sugar, potato starch, cocoa powder, baking soda, and xanthan gum in a medium bowl. Add egg, sour cream, milk, and vegetable oil. Whisk until batter is smooth. Allow batter to stand for five minutes. Lightly coat pan with nonstick cooking spray. Fill cavities about halfway with batter.

Bake until doughnuts spring back to the touch, about ten minutes. Turn doughnuts out onto a wire rack to cool. Repeat with remaining batter. Allow doughnuts to cool completely. Place a wire rack into a parchment-lined baking pan. Whisk together powdered sugar, milk, vanilla, and lemon extract in small bowl. Dip doughnuts, one at a time, into glaze. Shake off excess glaze. Place dipped doughnuts onto wire rack. Store doughnuts covered at room temperature for up to three days