Pour fresh apple cider into a saucepan and heat over medium until reduced to about 1/3 cup, around 15-20 minutes. Stir occasionally and cool completely.

In a large bowl, whisk together all-purpose flour, cornstarch, kosher salt, baking powder, baking soda, ground cinnamon, and freshly grated nutmeg.

In a separate bowl, beat together room temperature unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.

Add sour cream and vanilla extract, mixing well, then beat in eggs one at a time until smooth.

Gradually mix dry ingredients with the wet mixture, alternating with cooled reduced cider, until just combined. Avoid overmixing.

Cover bowl with plastic wrap and refrigerate dough for at least 2 hours, or overnight for best results.

Once chilled, roll dough to about 1/2 inch thickness and cut out donuts using a donut cutter.

Mix granulated sugar with ground cinnamon in a medium bowl for coating.

Heat canola oil in a deep skillet to 360°F. Fry donuts for 2-3 minutes on each side until golden brown and drain on paper towels.

Toss warm donuts in cinnamon sugar until evenly coated and enjoy immediately.