Simmer apple cider in a saucepan over medium-high heat for 20-25 minutes until reduced to exactly 3/4 cup. Reserve 2 tablespoons for the glaze and allow the remaining reduction to cool.
Preheat oven to 350°F (177°C). Generously grease a 10-to-12-cup bundt pan with softened butter and dust thoroughly with flour, tapping out excess.
In a large bowl, whisk together all-purpose flour, baking powder, baking soda, sea salt, 2 teaspoons cinnamon, nutmeg, and allspice.
In a stand mixer, beat 1 cup softened butter, dark brown sugar, and 1 cup granulated sugar on medium-high speed for 3-4 minutes until light and fluffy.
Add eggs one at a time, beating well after each, then blend in 1 1/2 teaspoons vanilla extract.
With mixer on low, add flour mixture in three additions, alternating with buttermilk and cooled reduced apple cider in two additions, mixing just until combined.
Pour batter into prepared bundt pan, smooth the top, and bake for 50-55 minutes until a toothpick inserted in the center comes out clean.
Cool cake in the pan for exactly 15 minutes, then invert onto a wire cooling rack.
Whisk powdered sugar, melted butter, remaining 1/2 teaspoon vanilla, and the 2 tablespoons reserved apple cider reduction into a glossy glaze. Drizzle over cake, sprinkle with cinnamon sugar (2 tablespoons sugar mixed with 1/2 teaspoon cinnamon), slice, and enjoy!