Allow the puff pastry to thaw according to the package directions, if it has not already been thawed.
Preheat the oven to 400°F.
Line a baking sheet with parchment paper.
Place the thawed puff pastry sheets on a lightly floured work surface.
Carefully unfold each puff pastry sheet.
Cut along the folds of each pastry sheet.
Cut each folded third in half to form rectangular pieces.
Transfer the pastry rectangles to the prepared baking sheet.
Leave at least 1 inch of space between each pastry rectangle.
Lightly brush the top of each pastry rectangle with the beaten egg.
Sprinkle the tops evenly with coarse sugar crystals or decorator sugar.
Place the baking sheet in the preheated oven.
Bake for 5 to 7 minutes, or until the pastries are puffed and lightly browned.
Remove the pastries from the oven.
Allow the pastries to cool completely before filling.
While the pastries are cooling, prepare the cream filling.
Place the softened butter and butter-flavored shortening in a mixing bowl.
Beat until the mixture is light and fluffy, approximately 5 minutes.
Add the powdered sugar, 1 cup at a time, mixing well after each addition.
Add the vanilla extract and mix until fully incorporated.
Add the marshmallow fluff.
Beat for approximately 2 minutes, or until the filling is smooth, light, and fluffy.
Once the pastries are completely cool, slice each pastry rectangle in half horizontally using a serrated knife.
Spread or pipe the cream filling onto the bottom half of each pastry.
Replace the top half of each pastry over the filling.
Serve immediately, or store as directed.