Preheat oven to 350F. Line two baking sheets with parchment paper.
In a mixing bowl using an electric mixer, cream together butter and both sugars. Add the vanilla and egg. In a separate bowl, mix together all dry cookie ingredients (cocoa powder, baking soda, salt and flour). Add the dry ingredients into the wet ingredients and mix until combined.
Scoop out 1 tablespoon rounds of the cookie dough and use your hands to form into balls. (You can make these bigger or smaller depending on your preference. They don't change size much when you bake). Set evenly on the prepared baking sheets. Bake for 7-9 minutes. (I like to check mine early to prevent over-baking). Remove and let cool completely.
In a separate mixing bowl, make the filling. Click the link above for the recipe.