Preheat oven to 350° F.
In a large bowl, sift together 2 cups flour, ½ cup cocoa, ¾ tsp salt and ¾ tsp baking soda.
In the bowl of a stand mixer fitted with the paddle attachment (or using electric beaters), cream 8 ounces butter, 1 ¼ cup brown sugar, and 1 tbsp molasses until soft and well combined, 1-2 minutes.
Add 1 egg and 2 tsp vanilla; mix well.
Add dry ingredients and fold in gently on low speed just until combined.
Pour about ¼ cup sugar into a small bowl. Scoop cookie dough into balls using a small spring-loaded scoop, about 1 ½ tablespoons. Roll each ball in sugar, tossing to coat, then place in mini muffin tin. Press down lightly on each cookie dough ball to flatten. Bake for about 12-15 minutes, until tops spring back when lightly pressed.
As soon as they come out of the oven, use a shot glass to indent each cookie. Gently press the shot glass into each cookie until you feel a little resistance – you should end up with about a 1/2" indent.
Allow to cool completely and then remove cookies from the muffin tin.