Pour 1.5 cups unfiltered apple cider into a small saucepan over medium-high heat. Bring to a rapid simmer and reduce for 12 to 15 minutes, stirring occasionally, until it concentrates down to 1/2 cup of deep amber apple syrup. Remove from heat and allow to cool for 10 minutes.

Preheat your oven to 350°F (175°C) and grease a 12-cavity donut pan thoroughly with non-stick cooking spray or melted butter. In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, allspice, and sea salt. In a separate bowl, whisk granulated sugar, melted butter, egg, buttermilk, vanilla, and the cooled reduced apple cider until smooth.

Pour wet ingredients into dry ingredients and fold gently with a silicone spatula just until incorporated (do not overmix). Spoon the thick batter into a large zip-top or piping bag, snip off the corner, and pipe the batter evenly into the prepared donut cavities, filling each about 3/4 full.

Bake at 350°F (175°C) for 10 to 12 minutes until the tops spring back lightly when pressed with a fingertip and a toothpick inserted into the center comes out clean. Cool in the pan for 5 minutes, then invert onto a wire cooling rack.

In a shallow bowl, whisk granulated sugar, cinnamon, and a pinch of salt. While donuts are still slightly warm, brush all sides generously with melted butter, then toss and roll in the cinnamon sugar mixture until thoroughly encrusted. Serve warm with hot spiced cider or coffee!