Pour apple cider into a small saucepan and bring to a boil over high heat. Reduce heat to medium-high and simmer vigorously until reduced to 1/2 cup, about 15 to 20 minutes. The cider should be thick and syrupy. Pour into a measuring cup and let cool completely. This concentrated cider is the key to real apple flavor.

In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg until well combined.

In a medium bowl, whisk together the cooled reduced cider, melted butter, sugar, eggs, and buttermilk until smooth.

Pour wet ingredients into dry ingredients and stir with a wooden spoon just until a shaggy dough forms and no dry flour remains. Do not overmix. The dough will be soft and slightly sticky. Cover and refrigerate for 10 minutes to firm up slightly.

Turn dough out onto a well-floured surface and pat into a 1/2-inch thick round. Flour your donut cutter generously and cut out donuts, flouring between cuts. Gather scraps, pat together once more, and cut remaining donuts. You should get 10 to 12 donuts plus holes. Place cut donuts on a floured parchment-lined baking sheet.

Pour oil into a large heavy pot to a depth of 2.5 inches. Heat over medium-high heat until oil reaches 350°F on a candy thermometer. Adjust heat to maintain temperature.

Carefully lower 2 to 3 donuts into the oil. Fry until deep golden brown on underside, about 1.5 to 2 minutes. Flip with a slotted spoon and fry until other side is golden, about 1 minute more. Transfer to a wire rack set over a baking sheet. Return oil to 350°F between batches. Fry donut holes for about 1 minute total.

While donuts are still warm, whisk together sugar and cinnamon in a shallow bowl. Toss warm donuts in the mixture to coat completely. Serve warm or at room temperature the same day.