Position rack in the center of the oven and heat to 350°F. Lightly coat the cavity of a standard donut tray with non-stick cooking spray.
In a large bowl, mix the flour, sugar, cornstarch, baking powder, cinnamon, and salt.
In a small bowl, mix the buttermilk, egg, and melted butter.
Add the buttermilk mixture to the dry ingredients, and sitr just until blended and smooth.
Spoon the batter into a large pastry bag or a heavy-duty zipper-type plastic and snip off the corner. Pipe the batter into the prepared pan, filling about ¾ full. (You can carefully spoon the batter into the cavities.)
Bake until the donuts a toothpick inserted into the donuts comes out clean, 7 to 9 minutes. Set the pan on a wire rack for 5 minutes, then remove the donuts to the rack to cool.