Preheat the oven to 350ºF. Line a large cookie sheet with parchment.
In a large bowl, sift together the flour and cocoa. Add in the salt and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugar for several minutes, then add the egg and vanilla. Beat together, scraping down the bowl as needed, until the egg has disappeared.
With the mixer on low, slowly add in the flour mixture, and beat the cookie dough until all the flour is thoroughly mixed in.
Transfer to a clean surface, and knead a couple times before dividing the dough in 2 and wrapping with plastic wrap to chill for 30 minutes in the fridge.
Rub the surface of the cookie stamps in the dough to grease them up. Scoop out tablespoon (20 g) balls of cookie dough and place on the parchment lined baking sheet. Dip the cookie stamp in granulated sugar to coat it, then stamp each of the cookies to press and emboss them.
Bake the cookies for 16–18 minuntes until they look dry on the surface.
Let cool 2 minutes before transferring to a wire rack to cool completely.