Line baking sheets with parchment paper or lightly spray with cooking spray.
In a small bowl, sift together the dry ingredients.
Beat the butter, shortening and sugar in a stand mixer until completely mixed together on low. Turn the mixer up to medium and beat for about 3 minutes.
Add egg and peppermint extract and beat for about two more minutes.
Add half of the flour mixture and half of the milk and beat on low until incorporated. Repeat with remaining flour and milk and beat until combined.
Using a small cookie scoop, drop the batter onto a baking sheet leaving two inches in between each scoop of dough.
Bake for about 10 minutes or until pies spring back when pressed gently.
Remove from oven and cool for about five minutes before transferring them to a rack to cool completely.
Cream together the butter and 2 cups of sugar until pale and fluffy, 2 to 3 minutes. This is where a stand mixer really comes in handy.
Add marshmallow fluff and vanilla extract. If you like you can use more peppermint extract instead of the vanilla to flavor the cream filling.
Beat on medium until everything is mixed together well and there are no visible lumps of butter.
Check the consistency of your filling. Dip a spoon in and take a big scoop of filling. Turn the spoon sideways. If the filling slides off the spoon add another half cup of the confectioners’ sugar. Keep adding the remaining cup of powdered sugar until your filling is stiff.
How to fill the Whoopie Pies
Fill the completely cooled whoopie pie cookies using a small cookie dough scoop or a tablespoon,
Place a scoop of filling into the center of one cookie. Sandwich the other cookie on top and gently push the cookies together until the filling almost reaches the edge of the cookie.
Using a small spoon sprinkle come Christmas sprinkles along the edges of the filling.
Place the filled whoopie pies on a baking sheet. Place the baking sheet in the refrigerator to chill the filling and allow it to set up.
As you fill each whoopie pie and decorate it put it on the cookie sheet in the fridge. Do not try filling the whoopie pies in an assembly line fashion. Fill each pie one at a time and chill.
If you decide to substitute peppermint for the vanilla only add ½ teaspoon of peppermint extract and taste.
Peppermint extract can be very overpowering if you use too much. Start out small and adjust. I wouldn't use more than a teaspoon though.