In a small saucepan, bring 1.5 cups of fresh apple cider and the cinnamon stick to a gentle boil over medium heat. Simmer uncovered for 15 to 20 minutes until reduced down to exactly 1/2 cup. Remove from heat, discard the cinnamon stick, and let the concentrated cider cool to room temperature (about 10 minutes).

Preheat your oven to 350°F (177°C). Lightly grease two 6-cavity standard donut pans with non-stick baking spray or melted butter. In a medium bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, nutmeg, allspice, and salt.

In a large bowl, whisk together the melted unsalted butter, packed light brown sugar, and granulated sugar until well combined. Whisk in the eggs one at a time, followed by the pure vanilla extract, buttermilk, and the cooled reduced apple cider until smooth.

Pour the dry ingredients into the wet mixture and fold gently with a rubber spatula just until combined and no flour streaks remain (do not overmix, which makes baked donuts dense). Transfer the batter into a large gallon zip-top or piping bag and snip the corner off. Pipe batter evenly into the prepared donut cavities, filling each about 3/4 full.

Bake for 10 to 12 minutes at 350°F until the tops spring back lightly when pressed with a fingertip and a toothpick inserted comes out clean. Let donuts cool in the pan for 5 minutes, then invert onto a wire cooling rack.

While the donuts are warm, melt 6 tablespoons of butter in a small shallow dish. In a separate wide bowl, whisk together 3/4 cup granulated sugar and 1.5 tablespoons ground cinnamon. Gently dip both sides of each warm donut into the melted butter, let excess drip off, then immediately press and roll generously in the cinnamon sugar mixture until completely coated in sparkling crystals. Serve warm and enjoy!