Preheat the oven to 350°F. Grease and line a 9x5-inch loaf pan with parchment paper.
Pour the apple cider into a small saucepan. Bring to a gentle simmer over medium heat and cook until reduced to ½ cup, about 15–20 minutes. Remove from the heat and let cool.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt and cinnamon.
In a large bowl, beat the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2–3 minutes.
Beat in the eggs one at a time.
Mix in the vanilla extract.
Beat in the sour cream until smooth.
Slowly mix in the cooled reduced apple cider.
Add the dry ingredients and mix just until no dry streaks remain. Do not overmix.
Spread the batter evenly into the prepared loaf pan.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack.
While the bread is still warm, brush the entire loaf with the melted butter.
In a shallow bowl, stir together the granulated sugar and cinnamon.
Generously coat the top and sides of the loaf with the cinnamon sugar mixture, gently pressing it onto the buttered surface.
Cool completely before slicing.