Step 1: In a small saucepan over medium heat, simmer the apple cider for 15-20 minutes until it is reduced to exactly 1/2 cup. Set aside to cool completely.
Step 2: Preheat your oven to 350°F (175°C) and generously grease a 10-12 cup Bundt pan with non-stick spray or butter and flour.
Step 3: In a medium bowl, whisk together the flour, baking powder, baking soda, 1.5 tsp cinnamon, nutmeg, and salt. Sift the mixture for a lighter cake.
Step 4: In a large stand mixer bowl, cream the room-temperature butter with granulated sugar and brown sugar for 3-5 minutes until light and fluffy.
Step 5: Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Step 6: Slowly whisk the cooled, reduced apple cider into the buttermilk. With the mixer on low, add 1/3 of the dry ingredients, followed by 1/2 of the cider-buttermilk mixture. Repeat, finishing with the last of the flour.
Step 7: Pour the batter into the prepared Bundt pan and smooth the top with a spatula.
Step 8: Bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 15 minutes.
Step 9: While the cake is slightly warm, invert it onto a wire rack. Combine the coating sugar and cinnamon in a small bowl. Brush the entire cake with melted butter and immediately toss the cinnamon sugar over it, pressing gently to adhere.