In a small saucepan set over medium-low heat, add apple cider and cook, stirring occasionally, until it has reduced to ½ cup, about 35 to 40 minutes. Set aside to cool.

Preheat the oven to 350ºF. Line a 9x13-inch metal baking dish with parchment paper and grease the parchment with baking spray. Set aside.

Melt the butter in a small saucepan set over medium-low heat, swirling often. Continue to cook until the butter is a deep golden hue and smells nutty, about 6 to 8 minutes. Set aside to cool.

In a large mixing bowl, whisk together the flour, cinnamon, nutmeg, allspice, cloves, baking soda, and salt. Set aside.

In a separate mixing bowl, whisk together the melted, browned butter and brown sugar until smooth and shiny. Add the egg and egg yolks, one at a time, and whisk after each addition. Add the apple slices, vanilla extract, and ½ cup apple cider reduction and whisk to combine.

Working in batches, fold in the flour mixture until combined. Transfer to the prepared baking dish.

Bake until a toothpick inserted in the center comes out clean, about 18 to 20 minutes. While the blondies bake, prepare the caramel fudge.

In a large saucepan set over medium heat, combine condensed milk, brown sugar, milk, butter, vanilla extract, and salt. Cook, whisking frequently, until the mixture turns a light amber brown and thickens significantly, about 15 to 18 minutes. Remove the mixture from the heat and continue to occasionally mix for 5 to 6 minutes, until it has cooled slightly.

Carefully pour the caramel fudge directly over the apple blondies. Sprinkle over a few pinches of flaky salt. Chill until the caramel has set, about 45 minutes.

Cut into 16 medium squares or 32 small rectangles.