Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside. In a medium saucepan set over medium-high heat, bring the apple cider to a boil. Once boiling, reduce the heat to medium and simmer until the cider is reduced to 1/4 cup. This should take approximately 30 to 35 minutes. Set the reduced cider aside to cool completely before using it in the batter.

In a medium bowl, sift together the flour, baking powder, baking soda, salt, cinnamon, ginger, and nutmeg. Set aside.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat 6 tablespoons of the room-temperature butter with the granulated sugar and brown sugar on medium speed until well combined. Scrape down the sides of the bowl, then add the vanilla extract and eggs, one at a time, mixing after each addition. Pour in the cooled reduced apple cider, followed by the apple butter or applesauce. Mix until smooth.

With the mixer on low speed, gradually add the dry ingredients to the wet ingredients and mix until just combined—do not overmix.

Scoop the dough onto the prepared baking sheets, using about 1 1/2 tablespoons of batter per cookie. Space the mounds about 3 inches apart. Bake for 11 to 13 minutes, rotating the pans halfway through, until the cookies are puffed and lightly spring back when touched.

Remove the cookies from the oven and allow them to cool on the baking sheets for 5 minutes. Then transfer them to a wire rack. While the cookies are still warm, brush the tops with the reserved 2 tablespoons of melted butter and sprinkle with the cinnamon sugar mixture, or gently toss them in the cinnamon sugar to coat. Allow cookies to cool completely before assembling.

To assemble, spread or pipe about 1 1/2 tablespoons of either Bourbon Caramel Buttercream or Brown Sugar Cream Cheese Icing onto the flat side of half the cookies. If desired, add a teaspoon of Bourbon Caramel Sauce on top of the filling. Top with a second cookie to form a sandwich.

For best texture and structure, refrigerate the assembled whoopie pies for 30 minutes before serving.

Bourbon Caramel Buttercream (Optional Filling)

In the bowl of a stand mixer fitted with the whisk attachment, beat the butter and a pinch of salt on medium speed for 2 to 3 minutes until light, smooth, and fluffy. Add the Bourbon Caramel Sauce and mix until fully incorporated. Scrape down the sides of the bowl. With the mixer on low, gradually add 1 cup of the sifted powdered sugar and mix until smooth. Add more powdered sugar as needed to achieve your desired consistency. If the buttercream is too thin, add additional powdered sugar; if it becomes too thick, add a splash of milk or heavy cream to loosen it.

Brown Sugar Cream Cheese Icing (Optional Filling)

In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese and butter on medium speed for 1 minute until smooth and fully combined. Add the brown sugar and mix for another minute until well incorporated. Mix in the vanilla extract. With the mixer on low, gradually add the sifted powdered sugar and continue mixing until the icing is thick and spreadable. If needed, add a little more powdered sugar to reach your preferred consistency. Use a piping bag or small spatula to spread the icing onto one half of each whoopie pie, then top with a second cookie.