Cream the butter until light and fluffy (about five minutes). Add powdered sugar and continue to mix another three minutes. Stir in the vanilla, cocoa, egg white, and salt and mix until the cocoa powder is integrated and the batter is smooth and creamy and the consistency of a thick frosting. Add the flour and mix just until the batter is no longer dusty looking. It might still be a bit crumbly – that’s okay. Turn the dough out onto a floured work surface, gather it into a ball, and knead it just once or twice to bring it together into a smooth mass. Divide into two pieces place each into a plastic bag and flatten into disks roughly 1 inch thick. Place in the refrigerator for 20 minutes to chill then remove dough one package at a time.

Preheat the oven to 350 degrees, with racks positioned in the middle of the oven, and line two baking sheets with parchment paper.

On a well-floured surface roll out dough very thin (remember these are going to be sandwich cookies) – roughly ⅛ inch thick. Or you might find it easiest to roll it out between pieces of plastic wrap. Stamp out cookies in whatever shape you like using 1-½ inch size cookie cutter. Place on prepared baking sheets and bake for 7-10 minutes, or until they smell of deep warm chocolate with toasty overtones.