For the Whoopie Pies:

• 2 ½ cups all-purpose flour
• 1 tsp baking soda
• 1 tsp baking powder
• ½ tsp salt
• 1 tsp ground cinnamon
• ½ tsp ground nutmeg
• ¼ tsp ground cloves
• ½ cup unsalted butter, softened
• 1 cup light brown sugar, packed
• 1 large egg, room temperature
• 1 tsp vanilla extract
• 1 cup apple cider, reduced to ¼ cup (boil down 1 cup until it reduces)
• ½ cup whole milk

For the Apple Cider Buttercream:

• ½ cup unsalted butter, softened
• 3 cups powdered sugar
• ¼ cup reduced apple cider (see note above)
• 1 tsp vanilla extract
• Pinch of salt

For the Caramel Filling:

• ½ cup store-bought caramel sauce (or homemade if you prefer)
• Pinch of flaky sea salt (optional)