Reduce the apple cider by pouring 4 cups into a large saucepan and bringing it to a gentle boil over medium heat. Simmer uncovered, stirring occasionally, until it reduces to 1 cup (about 30-40 minutes). Cool completely.
Prepare the dry ingredients by whisking together the flour, sugar, baking powder, baking soda, cinnamon, salt, nutmeg, and allspice in a large bowl until well combined.
Mix the wet ingredients in a separate medium bowl by whisking the eggs, melted butter, and reduced apple cider until smooth.
Combine wet and dry mixtures by pouring the wet ingredients into the dry ingredients. Gently fold together using a spatula just until a dough forms – avoid overmixing or your donuts will be tough.
Chill the dough by covering the bowl with plastic wrap and refrigerating for at least 30 minutes (or up to 2 hours). This makes the dough easier to handle and enhances flavor development.
Prepare for frying by filling a Dutch oven or heavy pot with 2 inches of oil. Heat to 350°F, using a candy thermometer to monitor the temperature. Meanwhile, line a baking sheet with paper towels and mix your cinnamon-sugar coating in a shallow bowl.
Roll out the dough on a well-floured surface to ½-inch thickness. Use a floured 3½-inch donut cutter (or a 3½-inch round cutter and a 1-inch cutter for centers) to cut out donuts and holes.
Fry the donuts in batches of 2-3, being careful not to overcrowd the pot. Cook for 1-2 minutes per side until deeply golden brown. Monitor your oil temperature between batches, adjusting heat as needed to maintain 350°F.
Coat while warm by removing donuts with a slotted spoon, briefly draining on paper towels (about 30 seconds), then tossing in the cinnamon-sugar mixture while still hot for maximum adhesion.
Serve immediately for the best flavor and texture experience, though they’ll remain delicious for up to 2 days stored in an airtight container.