Reduce the apple cider by half, then cool it completely to prevent batter from warming too soon. You should see the cider thicken slightly and measure about 1/4 cup.

Preheat the oven to 350F, then grease a 9x5 loaf pan. Ensure the pan is coated so the bread releases cleanly.

Whisk the all-purpose flour, baking powder, cinnamon, nutmeg, allspice, and salt in a bowl until evenly combined. The mixture should look uniform in color with no spice clumps.

Whisk the melted unsalted butter, granulated sugar, brown sugar, eggs, vanilla, reduced cider, and sour cream until smooth. The batter should be glossy and fully blended.

Fold the flour mixture into the wet ingredients just until no dry streaks remain, then pour the batter into the loaf pan. Stop mixing as soon as the batter is combined to keep the crumb tender.

Bake for 60-65 minutes at 350F until a toothpick comes out clean. The top should be golden and the center should spring back lightly.

Remove the loaf from the pan while warm. Let it cool just enough to handle so the crust coating sticks.

Brush all sides with melted unsalted butter, then roll or sprinkle generously with cinnamon sugar crust. The loaf should look fully coated with an even, cinnamon-speckled crust.