Prepare Apple Cider Reduction: Pour 1 cup of apple cider into a saucepan and bring it to a gentle simmer over medium heat. Stir occasionally and reduce it until only ¼ cup remains, about 10-15 minutes. Remove from heat and let it cool completely.

Make Cookie Batter: In a large mixing bowl, cream together ½ cup of softened unsalted butter and 1 cup of brown sugar until light and fluffy. Add in 1 large egg and 1 teaspoon of vanilla extract along with the cooled apple cider reduction. Mix until smooth.

Mix Dry Ingredients: In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, ½ teaspoon of salt, and 1 teaspoon each of cinnamon, nutmeg, and cloves.

Combine Mixtures: Gradually add the dry ingredients to the butter mixture, alternating with ½ cup of whole milk. Mix just until combined. Batter should be thick and slightly sticky.

Drop Batter: Prepare baking sheets with parchment paper. Drop rounded tablespoons of batter onto the sheets, spacing them 2 inches apart. Aim for about 24 cookies.

Bake: Preheat oven to 350°F (175°C) and bake for 10-12 minutes, until lightly golden and set. Cool completely on a wire rack.

Prepare Buttercream Filling: In a clean bowl, cream ½ cup of softened butter until smooth. Gradually add 2 cups of powdered sugar, then 2 tablespoons of cooled apple cider reduction and 1 teaspoon of vanilla extract until fluffy.

Assemble Whoopie Pies: Pipe buttercream onto the flat side of one cookie, drizzle with caramel sauce, and place another cookie on top. Chill in the refrigerator for 20 minutes before serving.