Reduce the Cider: Pour 1.5 cups apple cider into a small saucepan over medium-high heat. Simmer gently for 15 to 18 minutes until concentrated down to exactly 1/2 cup. Remove from heat and cool for 10 minutes.

Prep the Pan & Oven: Preheat oven to 350°F (175°C). Generously grease a 10 to 12-cup Bundt pan with nonstick baking spray (with flour) or brush with butter and dust lightly with flour, tapping out excess.

Whisk Dry Ingredients: In a medium bowl, whisk all-purpose flour, baking powder, baking soda, ground cinnamon, nutmeg, allspice, and fine salt until completely uniform.

Cream Butter & Sugars: In a large mixing bowl or stand mixer fitted with paddle attachment, beat softened butter, granulated sugar, and brown sugar on medium-high speed for 3 to 4 minutes until light, pale, and fluffy.

Add Eggs & Liquids: Beat in eggs one at a time, followed by pure vanilla extract. Whisk together the reduced cider and buttermilk in a measuring cup. With mixer on low speed, add dry ingredients in three additions alternating with cider mixture, starting and ending with dry. Mix just until combined without overmixing.

Bake to Golden Perfection: Spoon thick batter into prepared Bundt pan and smooth top evenly. Bake at 350°F (175°C) for 42 to 48 minutes until a toothpick inserted into center comes out clean. Let cool in pan for 15 minutes, then invert onto a wire cooling rack.

Brush & Sugar Coat: While cake is still pleasantly warm, stir granulated sugar and cinnamon together in a shallow bowl. Generously brush entire outside of cake with melted butter, then spoon and press cinnamon sugar evenly over all ridges until completely encrusted.