Preheat the oven to 325 degrees F. Butter or lightly spray a 9x5 loaf pan. Line the loaf pan with parchment paper leaving an overhang to lift the bread out easier.
In a small saucepan bring the cider to a boil over medium-high heat. Boil for about 10 minutes, or until it’s reduced to 3/4 cup. Pour 1/2 cup of the cider into a small bowl and set aside to cool for about 5 minutes. (Reserve 1/4 cup cider for later use.) Stir the sour cream and vanilla extract into the 1/2 cup cooled cider reduction.
In the same saucepan melt 8 tablespoons of butter. Let cool slightly.
In a medium size bowl, whisk together the flour, cornstarch, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt.
In a large bowl beat the eggs and 3/4 cup of sugar until pale and frothy, about 2 minutes. You can use a whisk, electric mixer, or stand mixer.
While whisking constantly, gradually stream in the butter. Continue to whisk until fully incorporated.
Whisk the dry ingredients into the egg mixture, alternating with the apple cider/sour cream mixture, beginning and ending with the dry ingredients. Mix just until combined, do not overmix.
Pour the cake batter into the prepared pan and bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Transfer the pan to a wire rack, and poke holes in the top of the cake with a toothpick. Drizzle 3 tablespoons of the reserved cider over the stop and let cool for 10 minutes.
For the sugar coating, mix a remaining 1/4 cup of sugar, 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, 1/4 teaspoon allspice, and a pinch of salt together in a small bowl.
Melt 1 tablespoon of butter and add to the remaining 1 tablespoon of reduced cider.
Lift the cake out of the pan and onto the wire rack. Set the rack inside a rimmed baking sheet. Peel the parchment away from the sides. Brush the melted butter mixture over the top and sides of the cake. Sprinkle the top and sides of the loaf generously with the sugar coating. (Using the parchment paper to help rotate the cake and collect excess sugar.)
Let cool completely before slicing.