You will need to keep a liquid measuring cup handy and pour the cider back into the measuring cup to be sure it has reduced enough. or me it usually takes about 10 minutes.
In a separate bowl (I like to use a large measuring cup), whisk together reduced apple cider, egg, and vanilla extract. Stir in melted butter (this will separate from the cider, that’s fine).
Pour liquid ingredients into dry ingredients and use a spatula to gently stir everything together until just combined (don’t over-mix or your donuts will be dense and dry).
Spoon or pipe batter evenly into prepared donut pan (I like to pour the batter into a large piping bag or large Ziploc bag and then just snip off one corner for easy piping).
Transfer to 350F (175C) oven and bake for 9-10 minutes or until a toothpick inserted in the thickest part of the donut comes out clean or with a few moist crumbs.
Whisk together granulated sugar and cinnamon in a shallow dish . Once donuts have cooled in the pan for 5 minutes then carefully remove to a cooling rack.