In a small saucepan over medium heat, bring 1.5 cups of fresh apple cider to a gentle simmer. Simmer uncovered for 15 to 20 minutes until reduced down to exactly 1/2 cup of concentrated, dark amber cider syrup. Set aside to cool to room temperature (this step concentrates pure orchard flavor).

Preheat your oven to 350°F (175°C). Grease two 6-cavity donut pans thoroughly with nonstick baking spray or melted butter.

In a medium bowl, whisk together all-purpose flour, baking powder, baking soda, ground cinnamon, freshly grated nutmeg, allspice, and salt until evenly distributed.

In a separate large bowl, whisk together melted butter, room-temperature egg, brown sugar, granulated sugar, buttermilk, vanilla extract, and the cooled reduced apple cider until smooth and glossy.

Pour the dry ingredients into the wet mixture and fold gently with a silicone spatula just until combined and no flour pockets remain. Do not overmix, which ensures an ultra-tender cake crumb.

Transfer batter into a large zip-top or piping bag and snip off the corner. Pipe evenly into the prepared donut cavities, filling each about 3/4 full.

Bake at 350°F for 10 to 12 minutes until donuts spring back lightly when pressed with a fingertip. Cool in the pan for 5 minutes, then turn out onto a wire rack.

While donuts are still pleasantly warm, brush both sides generously with melted butter (or dip quickly), then press and roll completely in the cinnamon sugar mixture until heavily coated. Serve immediately warm!