1) Preheat the oven to 375°F. Line two baking sheets with parchment paper, or spritz well with cooking spray.
2) In a small bowl or measuring cup, whisk together the eggs, buttermilk, vanilla and applesauce and spice until well combined.
3) Place the flour,sugar, baking powder, salt, baking soda, butter, and shortening in the bowl of your mixer. Add half of the apple/egg/buttermilk mixture. Beat on medium speed for two minutes.
4) Add the remaining liquid and beat for another two minutes.
5) Drop the batter onto the parchment, leaving about 2" between cakes for room to spread. Use 1/4 cup measure for large whoopies, or 2 tablespoons for small cakes.
6) Bake in a 375°F oven for 14 to 16 minutes or until the edges of the cakes just begin to brown. Remove from the oven and cool completely before peeling the cakes off of the parchment paper.
7) While the cakes cool, combine the prepared buttercream icing with the boiled cider and cinnamon oil until smooth and creamy.
8) Match up the cooled whoopies in pairs. Spread a generous amount of filling on one half of the pair and sandwich with the second cake. Press lightly to join. Serve immediately. The cakes also freeze very well and are delicious icy cold.
9) Yield: 8 large whoopie pies, or 16 small pies