In a medium bowl, combine the flour, cocoa powder, and salt; whisk to blend, then set aside. Using a hand-held or stand mixer, cream the butter and powdered sugar until light and fluffy, about 2 minutes. Blend in the eggs and vanilla. Add the dry ingredients and, with the mixer on low speed, blend just until incorporated and no streaks remain. Form the dough into a ball, wrap tightly in plastic wrap, and refrigerate for about 2 hours.

When ready to bake, preheat the oven to 325 degrees F (170 C). Line baking sheets with silicone baking mats or parchment paper.

On a lightly floured work surface, roll out the dough to a thickness of about 1/4-inch. Use cookie cutters to form desired shapes, and transfer cut-outs to the prepared baking sheets.

Bake cookies for 10-12 minutes, just until set. Allow cookies to cool on the baking sheets for 5-10 minutes, then remove to a wire rack to cool completely.

To make the filling, beat butter with a hand-held or stand mixer on high speed until creamy. Beat in the marshmallow fluff or cream until smooth. Reduce mixer speed to low and add powdered sugar. Once sugar is incorporated, increase mixture speed to high and beat until fluffy.

To assemble, pair cookies into tops and bottoms. Spread 1 scant tablespoon of filling in the center of the bottom half of one cookie, and gently cover with a top half. Repeat, and enjoy!