Pour the apple cider into a small saucepan and bring to a boil over medium-high heat. Cook until reduced to 1/2 cup, about 15-20 minutes. Remove from the heat and let cool for 10 minutes.
1 1/2 cups apple cider
You can reduce the cider in advance and store in the fridge until ready to use.
Preheat the oven to 350°F. Grease a donut pan with baking spray or butter.
Whisk melted butter with the brown sugar in a large bowl until smooth. Add the egg and vanilla extract and whisk until well combined. Whisk in the reduced apple cider.
4 tablespoons unsalted butter, 1/2 cup packed light brown sugar, 1 large egg, 1 teaspoon vanilla extract
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and cloves.
Add the dry ingredients to the cider mixture and stir until no dry streaks remain. Do not overmix.
Divide the batter evenly among the prepared donut cavities, filling each about three-quarters full. Bake for 12-15 minutes, until the donuts spring back lightly when pressed and a toothpick inserted into the thickest part comes out clean.
Let the donuts cool in the pan for 5 minutes, then transfer them to a wire rack. Wipe out pan, re-grease, and bake with any remaining batter.
Brush the remaining 2 tablespoons melted butter over the warm donuts. Combine the granulated sugar and cinnamon in a shallow bowl, then toss each donut in the cinnamon sugar until evenly coated. Serve warm or at room temperature.