In the bowl of a mixer, beat the butter with the brown sugar and ¼ cup of the granulated sugar at medium speed until soft. Add the vanilla and the egg, reduce the speed, and incorporate the cocoa powder. Scrape the sides of the bowl if need be, add the salt and the flour, and continue beating until it is all thoroughly combined. The dough should be soft and a bit cakey. Gather into a ball, wrap in plastic and place in the fridge anywhere from 20 to 30 minutes.
Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper and set the oven racks in the upper and lower thirds of the oven.
Flour the countertop and a rolling pin generously. Roll out the dough into a rectangle of about 1/8” thick. Cut into rectangles of about 2” by 1½”. With the help of a metal spatula, place on the baking sheets. With the tip of a pencil or a toothpick, mark a “T” with 3 to 4 dots going up and then 1 to 2 on each side of the top of the line to mark a T. Sprinkle with the remaining 2 tablespoons of granulated sugar. You can reuse any scraps of dough and roll out again.
Bake anywhere from 7 to 8 minutes, until firm. Remove from the oven and let cool.