Reduce the Cider: In a small saucepan over medium heat, bring 2 cups of apple cider to a gentle boil. Simmer uncovered for 15 to 18 minutes until reduced to exactly 3/4 cup of deeply amber, concentrated syrup. Set aside to cool to room temperature.

Prep Pan & Whisk Dry Ingredients: Preheat oven to 350°F (175°C). Generously grease a 10 to 12-cup bundt pan with baking spray with flour or butter and flour. In a large bowl, whisk together all-purpose flour, baking powder, baking soda, ground cinnamon, nutmeg, allspice, and fine salt.

Cream Butter & Sugars: In a stand mixer fitted with the paddle attachment (or using an electric hand mixer), beat softened butter, granulated sugar, and brown sugar on medium-high speed for 3 to 4 minutes until light, fluffy, and pale in color.

Add Eggs & Wet Liquids: Beat in eggs one at a time, incorporating fully after each addition, then stir in vanilla extract. In a measuring cup, combine the cooled reduced cider with the room-temperature buttermilk.

Combine Batter & Bake: With mixer on low speed, alternate adding the dry flour mixture (in 3 additions) and the cider-buttermilk liquid (in 2 additions), beginning and ending with dry ingredients. Mix just until combined without overworking. Spoon thick batter into the prepared bundt pan, smooth the top, and bake for 42 to 48 minutes until a toothpick inserted into the center comes out clean.

Cool, Butter Brush & Sugar Coat: Cool cake in the pan on a wire rack for 10 minutes, then invert onto the rack. While the cake is still warm, generously brush all exterior fluted ridges with melted butter using a silicone pastry brush, then spoon and pat the cinnamon sugar mixture all over the cake until lavishly coated.

Glaze & Slice: Whisk powdered sugar, cider, vanilla, and cinnamon into a silky drizzling glaze. Drizzle over the cinnamon sugar ridges, let set for 10 minutes, and slice into generous bakery-style wedges!