Pour 1 cup of apple cider into a saucepan and simmer over medium heat until it reduces to 1/4 cup, which takes about 10-15 minutes. Set it aside to cool before using it in the batter and buttercream.

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 room temperature butter and brown sugar together until light and fluffy. Add the egg and vanilla extract, then beat until well combined. Slowly mix in the cooled apple cider reduction from Step 1.

Add the dry ingredients from Step 2 to the wet ingredients in three parts, alternating with the milk, beginning and ending with the dry mix. Mix until just combined – be careful not to overmix to keep the cakes tender. Scoop 2-tablespoon-sized portions of batter onto the prepared baking sheets, leaving about 2 inches between. Bake for 10-12 minutes, or until the cakes are firm to the touch and a toothpick comes out clean. Transfer to a wire rack to cool completely. I always let them cool fully before assembling – it keeps the buttercream from melting.

In a medium bowl, beat the room temperature butter until creamy. Gradually add the confectioners' sugar, mixing until fully incorporated. Beat in the cooled apple cider reduction (reserved from Step 1), vanilla extract, and a pinch of salt until smooth and fluffy. For the smoothest buttercream, I like to beat it for an extra two minutes at the end.

Pair up the cooled whoopie pie halves by size. Pipe or spoon a generous amount of buttercream from Step 4 onto the flat side of one whoopie pie half. Drizzle with caramel sauce and, if desired, sprinkle with flaky sea salt for crunch and contrast. Top with the second half to create a sandwich, pressing gently. Repeat with the remaining pies. I love adding a bit of flaky sea salt on top of the caramel – it really balances the sweetness.