Prep and Reduce
Preheat your oven to 375°F and grease a standard 12-cup muffin pan. Pour the apple cider into a saucepan and bring it to a boil over high heat. Let it boil until it has reduced by half (you should have about 1 cup left), which takes about 10 minutes. Remove from heat and allow to cool completely.

Mix Dry Ingredients
In a mixing bowl, stir together the all-purpose flour, corn starch, baking powder, baking soda, ground cinnamon, ground nutmeg, and kosher salt until well blended.

Cream Butter and Sugar
In a separate mixing bowl, cream together the room temperature butter, 1/2 cup granulated sugar, and brown sugar until the mixture is smooth and well blended.

Combine Wet Ingredients
Stir the vanilla extract and egg into the creamed butter mixture. Then, pour in the cooled reduced cider and blend until everything is well combined.

Make the Batter
Pour the dry flour mixture into the wet liquid ingredients. Stir several strokes until everything is just blended. Be careful not to over-mix, or the muffins may turn out tough.

Bake
Spoon the batter into the prepared muffin tin, filling each cup about 3/4 full. Bake uncovered for 15 to 17 minutes. You will know they are done when the tops are lightly browned and an inserted knife comes out clean. Let them cool in the pan for 10 minutes.

Coat the Muffins
While the muffins cool, make the topping by stirring together the 3/4 cup granulated sugar and 3 tablespoons cinnamon. Once the muffins have cooled enough to handle, brush each one all over with the melted butter, then roll them in the cinnamon-sugar mixture until well coated.