Stirring occasionally, boil the apple cider in a small saucepan over medium-high heat until you’re left with 1/2 cup. Start checking at 15 minutes, 20 minutes, 25 minutes, etc until you have 1/2 cup (120ml). Mine takes about 25 minutes.
Cool for 10 minutes before using in step 4. You can even reduce the apple cider the day ahead of time. Cover and refrigerate overnight, then bring to room temperature before using.
Preheat oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.
Whisk the flour, baking powder, baking soda, cinnamon, ginger, nutmeg, and salt together in a large bowl. In another large bowl using a hand mixer or a stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium-high speed until creamed, about 1 minute.
Add the oil, applesauce, eggs, and vanilla extract and beat on high speed until combined, about 1-2 minutes.
The mixture will look curdled as a result of the varying textures trying to combine. This is OK and it will come together when you add the dry ingredients.
Scrape down the sides and up the bottom of the bowl and beat again as needed to combine.
Add the dry ingredients to the wet ingredients, pour in the 1/2 cup of reduced apple cider, then mix on low until completely combined. Batter will be thick and creamy.
Using a medium cookie scoop, scoop mounds of batter, about 1 and 1/2 Tablespoons each, onto prepared baking sheets about 3 inches apart. If desired, sprinkle each with cinnamon sugar. See recipe note.
Bake the cookies for 12-14 minutes or until the edges are very lightly browned and the tops spring back when lightly touched. Mine usually take 13 minutes. Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Make the filling as they cool.
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until completely smooth and creamy, about 2 minutes.
Add the confectioners’ sugar, cinnamon, nutmeg, ginger, apple cider, and vanilla extract, then beat on medium speed until combined. Taste. Add a pinch of salt if desired (I always do). Add another Tablespoon of apple cider if needed to thin out or more spices if desired for extra flavor.
Pair the cookies up based on their size. Spread or pipe (I used Wilton 1A piping tip) the frosting onto the flat side of one cookie and sandwich with the other. Repeat with remaining cookies. Serve.
Cover leftover whoopie pies and store in the refrigerator for up to 1 week. They are EXCELLENT on day 2 because the flavor intensifies overnight.
