Preheat oven to 350 F. Generously butter then flour two 9x5 loaf pans.
In a mixing bowl, whisk together the flour, salt, allspice, nutmeg, baking powder, cloves, and cinnamon. In a measuring cup or separate bowl, stir together the cider and vanilla.
In a stand mixer on medium-high speed using the paddle attachment, beat together the butter and shortening until creamy. While still mixing, gradually add the sugar. Once all the sugar has been added, beat for a full 5 minutes. Mixture will be light and fluffy. Scrape down the bowl.
1 cup unsalted butter, softened, 1/2 cup butter flavored shortening, 21 ounces granulated sugar
Reduce mixer speed to medium. Add the eggs, 1 at a time, beating for 1 minute after each addition.
6 large eggs
Reduce mixer speed to low and add half of the dry ingredients to the butter mixture, mixing just until combined. Next add the cider mixture just until combined. Finish with remaining flour mixture just until combined. Scrape down the bowl to make sure everything is evenly mixed. Batter will be thick.
Pour batter into prepared pans and spread out evenly across the top. Tap the pans on the counter a couple times to ensure there are no air pockets.
Place pans on an insulated baking sheet instead of directly on the racks. This creates a heat barrier and helps the cakes bake more evenly. If you don't have an insulated pan, use two regular baking sheet stacked on top of each other.
Bake for 50-60, or until a knife inserted in the center comes out clean. Cool cakes in pans for ten minutes, then remove from pans and cool completely on wire rack.
For the Brown Butter Glaze
Line work surface with a sheet of waxed paper to catch drips, and place a wire rack over the top. Place loaves on rack.
Add powdered sugar to a medium bowl.
17 ounces powdered sugar
Place butter in a small saucepan. Melt butter over medium heat until bubbly. Cook, stirring constantly until golden brown. Remove from heat and pour butter into the bowl of powdered sugar.
1/2 cup unsalted butter
Add vanilla and milk, stir until smooth.
2 teaspoons vanilla extract, 4-6 tablespoons milk
Immediately drizzle glaze over the loaves. Allow to cool and set before serving.