To reduce the apple cider: Place the cider in a saucepan, bring to a boil, reduce and simmer uncovered over low heat, stirring occasionally, until the cider is reduced to 1/2 cup, about 20 minutes. Let cool. Prepare the topping by combining the sugar and cinnamon in a bowl and set aside until ready to use.
Preheat the oven 350 F. Lightly grease a donut pan.In a bowl, combine the flour, baking powder, baking soda, salt and spices. In a large bowl whisk together the sugars, butter, egg, vanilla extract and apple cider. Add the flour mixture and the buttermilk and whisk just until combined. Be careful not to over-mix. Spoon the batter into the prepared donut pan to 3/4 full. Bake on the center rack for 12-15 minutes or until the tops of the donuts are lightly browned and the donuts spring back slightly when you gently press them. Let them cool for about 2 minutes and then transfer them to a wire rack.
Preferably while the donuts are still a little warm, lightly spray them with oil or lightly brush them with some extra butter, and roll them in the cinnamon sugar.Serve immediately (recommended) or once fully cooled store in an airtight container at room temp for up to 2 days or in the fridge for up to 5 days. Warm them in the microwave for a few seconds before serving.