Simmer 1 1/2 cups of apple cider in a small saucepan over medium-high heat for 8–10 minutes until reduced to 1/2 cup of syrupy apple cider concentrate. Let cool for 5 minutes while preheating your oven to 350°F (175°C) and greasing two 6-cavity donut pans.

Whisk the flour, baking powder, baking soda, cinnamon, nutmeg, and salt in a medium bowl. In a large bowl, whisk the warm browned butter, eggs, dark brown sugar, granulated sugar, milk, vanilla, and reduced apple cider until smooth.

Fold the dry ingredients into the wet ingredients with a spatula just until no flour streaks remain.

Spoon the batter into a large zip-top bag, snip a 1/2-inch corner, and pipe evenly into the greased donut cavities (filling each 2/3 full). Bake for 10–11 minutes until the tops spring back lightly when touched.

Cool in the pans for 2 minutes, then invert onto a wire rack. Brush the warm donuts all over with melted brown butter and toss immediately in the cinnamon-sugar mixture until thickly coated.