Begin by preparing the apple cider reduction. Pour 1 cup of apple cider into a saucepan and simmer over medium heat until it reduces down to ¼ cup, which should take about 10 to 15 minutes. Once reduced, set it aside to cool while you prepare the whoopie pie batter.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, nutmeg, and cloves. In a large bowl, beat the butter and brown sugar until light and fluffy. Add the egg and vanilla extract, mixing until fully combined. Gradually stir in the cooled apple cider reduction. Add the dry ingredients in three parts, alternating with the milk, starting and ending with the dry ingredients. Mix just until combined to avoid overmixing. Using a cookie scoop or tablespoon, drop 2-tablespoon-sized mounds of batter onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 10 to 12 minutes, or until the cakes are firm to the touch and a toothpick inserted into the center comes out clean. Transfer the baked whoopie pie halves onto a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar, then the cooled apple cider reduction, vanilla extract, and a pinch of salt. Continue beating until the buttercream is smooth and fluffy.

Match the whoopie pie halves by size. Pipe or spoon a generous layer of the apple cider buttercream onto the flat side of one half. Drizzle a spoonful of caramel sauce over the buttercream. For added texture, sprinkle a pinch of flaky sea salt on top of the caramel. Place the second half of the whoopie pie on top, pressing gently to form a sandwich. Repeat the process with the remaining whoopie pies.