Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats to ensure your whoopie pie cookies don’t stick and bake evenly.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt. This evenly distributes the leavening agents and spices to create a balanced flavor and texture.
In a large bowl, use an electric mixer to cream the softened unsalted butter and brown sugar until light and fluffy, about 3-4 minutes. This process helps incorporate air for a tender crumb.
Beat in the large egg and vanilla extract until fully combined. These ingredients add moisture and richness to the batter.
Add the flour mixture in three parts, alternating with the apple cider, beginning and ending with the flour. Mix gently after each addition until just combined. Overmixing can cause tough cookies, so be careful to stop once the batter is smooth and uniform.
Using a cookie scoop or tablespoon, drop rounded spoonfuls of batter onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are set and the tops spring back when lightly pressed.
Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a cooling rack. Cooling completely is essential before assembling whoopie pies to prevent the filling from melting.
In a small bowl, whisk together the caramel sauce, powdered sugar, and milk until smooth and spreadable. Adjust with more milk or powdered sugar as needed to achieve a creamy but thick consistency perfect for sandwiching.
Pair cookies of similar size. Spread about a tablespoon of the caramel filling onto the flat side of one cookie, then gently sandwich with its match. Press just enough to spread the filling to the edges without squeezing it out.
Your Apple Cider Whoopie Pie Cookies with Caramel are best enjoyed fresh but can be stored for later indulgence.