In a small saucepan, simmer 2 cups apple cider over medium heat until reduced to 1/2 cup, about 15–20 minutes. Cool to lukewarm. This is your flavor bomb.
Heat oven to 400°F (205°C). Lightly grease a mini muffin tin or doughnut hole pan with oil or spray. Mix the coating sugar, cinnamon, and a pinch of salt in a shallow bowl; set aside.
In a large bowl, whisk whole-wheat pastry flour, all-purpose flour, granulated sugar, brown sugar, baking powder, baking soda, salt, cinnamon, and nutmeg until combined.
In a separate bowl, whisk reduced cider, buttermilk, egg, vanilla, and 2 tablespoons melted butter until smooth.
Pour wet into dry and stir gently until just combined. The batter should be thick and scoopable. Don't overmix—lumpy is okay.
Spoon or pipe batter into mini muffin cups, filling each about 3/4 full for rounded tops.
Bake 9–11 minutes, until the tops spring back and a toothpick comes out with a few moist crumbs. Don't overbake, unless you prefer hockey pucks.
Immediately brush warm tops with remaining 2 tablespoons melted butter. Roll in the cinnamon-sugar mixture while still warm so it sticks like a dream.
Let them rest on a rack for 10 minutes. They set up and the coating locks in.
Whisk 1/2 cup powdered sugar with 1–2 tablespoons cider to drizzle over the coated holes for extra gloss and flavor. Not mandatory, just dangerously good.