Reduce the Apple Cider: In a small saucepan over medium-low heat, simmer 1 1/2 cups of fresh apple cider for 15 to 20 minutes, stirring occasionally, until it concentrates down to exactly 1/2 cup. Set aside to cool to room temperature.
Preheat & Prep the Pan: Preheat oven to 350°F (175°C). Generously grease two standard 6-cavity donut pans with nonstick cooking spray or melted butter.
Whisk Dry Ingredients: In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and sea salt until evenly distributed.
Combine Wet Ingredients: In a separate bowl, whisk together melted cooled butter, egg, brown sugar, granulated sugar, milk, vanilla extract, and cooled reduced cider until smooth and glossy.
Fold into Batter: Pour the wet mixture into the dry ingredients. Use a rubber spatula to fold gently just until no pockets of dry flour remain. Do not overmix.
Pipe into Donut Pans: Spoon or transfer the batter into a piping bag or gallon zip-top bag with the corner snipped off. Pipe batter into the donut cavities, filling each about 3/4 full.
Bake to Golden Perfection: Bake at 350°F for 10 to 12 minutes, or until the tops spring back lightly when pressed and a toothpick comes out clean.
Dip in Cinnamon Sugar: Allow donuts to cool in the pan for 5 minutes, then invert onto a wire rack. While still warm, dip both sides of each donut into melted butter, then toss generously in cinnamon sugar. Serve warm!