Preheat the oven to 300°F. Lightly grease (or line with parchment) two baking sheets., In a large mixer bowl, cream the butter and sugar for 1 minute, just to bring them together., Combine the vanilla, baker's ammonia, and salt in a very small bowl. Stir to dissolve the ammonia; the salt won't fully dissolve. Combine the vanilla mixture with the butter and sugar, and beat until smooth., Sift together the flour and cocoa. Add to the butter mixture and mix on low speed. The mixture will seem dry at first, but continue to mix for 3 to 5 minutes, until the dough comes together., Break off pieces of dough about the size of a large gumball; a teaspoon cookie scoop works well here. Roll the pieces into balls. Space them on the prepared baking sheets, leaving 1 1/2" between them., Use the bottom of a glass, dipped in sugar if necessary to prevent sticking, to flatten the balls to about 3/8" thick. Try using other kitchen items, such as a slotted spoon or a potato masher, to give different imprints., Bake the cookies for 18 to 20 minutes, until the fragrance of chocolate begins to fill the kitchen. Transfer the cookies to a rack to cool. They will continue to crisp up as they cool. Store completely cooled cookies airtight for up to 5 days.