Gather all your ingredients and let the butter soften at room temperature.

In a mixing bowl, combine the softened butter and powdered sugar. Mix until it becomes light and fluffy, about 2-3 minutes.

Incorporate the marshmallow fluff into the butter mixture.

Stir in the vanilla extract and a pinch of salt.

Continue mixing until everything is well combined and fluffy.

Give the filling a taste and adjust sweetness if needed.

If using, transfer the filling to a piping bag for easy application.

Pipe or spread the filling between two cooled whoopie pie cookies.