5. Make the Filling: in the Bowl of a Stand Mixer Fitted with the Paddle Attachment, Beat Together the Powdered Sugar and the Butter, Starting on Low and Increasing to Medium Speed, until the Mixture is Crumbly, about 1 Minute. Add the Heavy Cream, Vanilla, Peppermint Extract, and Salt and Beat on High until Smooth, about 3 Minutes

Assemble the Whoopie Pies: Spread Filling onto the Flat Side of One Cake Using a Knife or Spoon. (Alternatively, You Can Use a Pastry Bag with a round Tip to Pipe the Filling onto the Cakes, Which Will Give You a Smoother, Neater Presentation.) Top It with Another Cake, Flat-side down. Repeat Wit the Rest of the Cakes and Filling. Roll the Edges in Crushed Candy Canes