Prepare Cider Reduction & Oven: Preheat your oven to 350°F (175°C). Generously grease and flour a 10 or 12-cup Bundt pan. In a small saucepan, bring the 2 cups of apple cider to a boil over medium-high heat. Reduce the heat and let it simmer until it has reduced to about 2/3 cup, which can take 15-20 minutes. Set aside to cool slightly.
Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, light brown sugar, baking powder, baking soda, 1 tsp cinnamon, nutmeg, and salt. Make sure there are no lumps of brown sugar.
Mix Wet Ingredients: In a separate medium bowl, whisk together the melted butter, eggs, buttermilk, vanilla extract, and the cooled apple cider reduction until well combined.
Combine Batter: Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can lead to a tough cake.
Bake the Cake: Pour the batter evenly into the prepared Bundt pan. Bake for 40-50 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. If the top begins to brown too quickly, you can tent it loosely with aluminum foil.
Cool and Prepare Topping: Let the cake cool in the Bundt pan on a wire rack for 15 minutes. While it cools, prepare the topping. In a small bowl, mix together the 1 cup of granulated sugar and 2 tsp of ground cinnamon.
Apply Topping: Carefully invert the warm cake onto the wire rack. Place a baking sheet under the rack to catch any drips. While the cake is still warm, brush it all over with the 1/2 cup of melted butter. Immediately and generously sprinkle the cinnamon-sugar mixture over the entire cake, pressing gently to help it adhere. The warmth of the cake is key to getting the topping to stick perfectly.
Serve: Let the cake cool for at least another 20-30 minutes before slicing and serving. This allows the topping to set. Enjoy the cozy flavors of fall!