Start by making the apple cider reduction. Pour 1 cup of apple cider into a saucepan and simmer over medium heat until it reduces to ¼ cup. This should take about 10-15 minutes. Once done, let it cool while you prepare the whoopie pie batter.

Preheat the 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 together until light and fluffy. Add the egg and vanilla extract, and beat until combined. Gradually mix in the cooled apple cider reduction. Add the dry ingredients in thirds, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined. Use a cookie scoop or tablespoon to drop 2-tablespoon-sized rounds of batter onto the prepared baking sheets, leaving about 2 inches between each. Bake for 10-12 minutes, or until the cakes are firm to the touch and a toothpick inserted into the center comes out clean. Transfer the whoopie pie halves to a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy. Gradually add in the powdered sugar, followed by the cooled apple cider reduction, vanilla extract, and salt. Beat until smooth and fluffy.

Pair up the whoopie pie halves by size. Pipe or spoon a generous amount of apple cider buttercream onto the flat side of one whoopie pie half. Drizzle a spoonful of caramel sauce over the buttercream. If you like a bit of crunch, sprinkle some flaky sea salt over the caramel. Top with the second half of the whoopie pie, pressing gently to create a sandwich. Repeat with the remaining pies.