Pour 2 cups of apple cider into a small saucepan and bring to a simmer over medium heat. Allow it to bubble gently until reduced to ½ cup, about 20-25 minutes. Let it cool completely before using.
Preheat your oven to 350°F (175°C). Generously grease a 10-12 cup bundt pan, making sure to coat all the nooks and crannies. A baking spray with flour works best for easy release.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, allspice, and cardamom if using.
In a large bowl using an electric mixer, beat the softened butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition. Mix in the applesauce, cooled cider reduction, and vanilla extract until well combined.
Gradually add the flour mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the flour (flour-buttermilk-flour-buttermilk-flour). Mix on low speed just until combined after each addition.
Pour the batter into the prepared bundt pan and smooth the top. Tap the pan gently on the counter a few times to release any trapped air bubbles. Bake for 50-55 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan for 10-15 minutes. Meanwhile, prepare the coating by mixing the sugar, cinnamon, and nutmeg in a shallow dish. Melt the butter in another bowl.
Carefully invert the cake onto a cooling rack. While still warm, brush the entire cake with melted butter, then immediately sprinkle and press the cinnamon-sugar mixture onto all surfaces.
Allow the cake to cool completely before slicing and serving.