Preheat oven to 350°F (175°C) and generously grease a bundt or tube pan.
Reduce fresh apple cider in a saucepan over medium heat until thickened and syrupy; let cool slightly.
Cream butter with granulated and brown sugars until light and fluffy, then beat in eggs one at a time followed by vanilla.
In another bowl, whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg together.
Alternate adding dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients. Stir in the cooled cider reduction.
Pour batter into prepared pan, smoothing the top evenly.
Bake for about 45–50 minutes or until a tester inserted in center comes out clean.
Let cool for 10 minutes, then invert onto a rack. Brush with melted butter and sprinkle evenly with cinnamon-sugar topping.
Cool completely before slicing to allow texture and flavor to settle beautifully.