In a medium saucepan, add 2 cups of apple cider and bring it to a simmer over medium heat. Allow it to cook for about 25-30 minutes, stirring occasionally, until it thickens and reduces to about ½ cup. Once done, remove it from heat and let the reduced cider cool completely before using it in the batter.

In a large mixing bowl, combine the cooled reduced apple cider with 1 cup of unsweetened applesauce and 1 cup of brown sugar. Using an electric mixer, beat the mixture on medium speed until it becomes light and fluffy, approximately 2-3 minutes.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, 1 teaspoon of baking soda, 1 teaspoon of cinnamon, ½ teaspoon of nutmeg, and ½ teaspoon of ginger. Gradually add this dry mixture to your wet ingredients, folding gently until just combined.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. Using a cookie scoop or spoon, drop rounded tablespoonfuls of dough onto the prepared sheets, leaving about 2 inches in between each scoop. Bake for 12-13 minutes, or until the edges are set and the tops are slightly springy. Let them cool completely on a wire rack.

In a mixing bowl, beat ½ cup of unsalted butter at room temperature until creamy and smooth. Gradually add 2 cups of powdered sugar, mixing well. Pour in the remaining reduced apple cider and 1 teaspoon of vanilla extract. Continue to beat until the buttercream is light, fluffy, and spreadable.

To fill your cooled whoopie pies, spread a generous layer of buttercream on the flat side of one cookie. Then, gently press another cookie on top to create a sandwich. Repeat this process with the remaining cookies until they are all filled.

For an extra touch of fall flavor, lightly dust the assembled Apple Cinnamon Spice Whoopie Pies with a sprinkle of cinnamon. This step is optional.