Reduce the apple cider: Stirring occasionally, simmer the apple cider in a small saucepan over low heat until you’re left with about ½ cup. Start checking at 10 minutes, and then keep an eye on it until you are left with a ½ cup. This usually takes about 20 minutes. If there are any spices or solids on top of your reduced apple cider, leave them. Set aside to cool for 10 minutes.
Preheat your oven to 350°F (175°C) and spray your donut pans with cooking spray.
In a large mixing bowl, whisk together the dry ingredients: 2 cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon cinnamon, and ¼ teaspoon salt.
In a separate medium bowl, combine the wet ingredients along with the sugar: ½ cup reduced apple cider, ½ cup granulated sugar, ¼ cup dark brown sugar, 1 large egg, ¼ cup melted butter, ¼ cup sour cream, and 1 teaspoon vanilla extract. Mix until smooth.
Pour the dry ingredients into the wet ingredients and stir until just combined. Be careful not to overmix - a few lumps in your donut batter are okay!
Transfer the batter to a disposable large piping bag or a ziplock bag with a corner snipped off. Pipe the batter into the prepared doughnut pans, filling each cavity about ⅔ full.
Bake for 10-12 minutes, or until the donuts are golden brown and spring back when lightly touched.
Let the donuts cool in the pan for 5 minutes, then transfer them to a wire rack.
Allowing the donuts to cool before coating makes them easier to handle and ensures they stay intact when tossed in the sugar.
Combine 1 cup sugar with 1 teaspoon cinnamon. Dunk both sides of the donuts in 6 tablespoons melted butter and then into the cinnamon sugar mixture.
For the best apple cider donuts, enjoy them while they're still warm! These homemade donuts can be stored in an airtight container at room temperature for 2-3 days, but they're at their absolute best fresh out of the oven.