01 - Preheat oven to 350°F (175°C). Lightly grease a donut pan using nonstick spray.
02 - In a medium saucepan over medium heat, simmer apple cider until reduced to 1/2 cup (120 ml), approximately 10 minutes. Allow to cool slightly.
03 - In a medium bowl, thoroughly whisk together all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, ground allspice, and salt.
04 - In a large bowl, whisk together melted unsalted butter, granulated sugar, and packed light brown sugar until incorporated. Beat in eggs one at a time. Incorporate the reduced apple cider, whole milk, and vanilla extract.
05 - Gently fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing the batter.
06 - Spoon or pipe the batter into the prepared donut pan, filling each cavity to about 75% capacity.
07 - Bake for 12 to 15 minutes, or until the donuts spring back when lightly touched and a toothpick inserted into the center comes out clean. Allow the donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
08 - For the glaze, whisk together sifted powdered sugar, whole milk, and the scraped vanilla bean seeds (or vanilla bean paste) in a bowl until a smooth, pourable consistency is achieved.
09 - Dip the completely cooled donuts into the prepared glaze, allowing any excess to drip off. Place the glazed donuts back on the wire rack until the glaze sets.
10 - For an optional cinnamon sugar coating, gently brush the warm donuts with melted unsalted butter, then toss them in a mixture of granulated sugar and ground cinnamon before proceeding with the glaze.