In a medium mixing bowl, whisk together the dry ingredients. Set aside.
In the bowl of a stand mixer, cream together the butter with the brown sugar and ¼ cup of granulated sugar on medium speed. The mixture should be light and fluffy.
With the mixer turned off, add the vanilla, egg, molasses and sourdough discard. Then, mix on medium speed until smooth.
On low speed, slowly add the flour mixture and mix until well incorporated. (Scrape the sides of the bowl with a rubber spatula once during the mixing process to ensure all the ingredients get well mixed.)
Cover with plastic wrap and refrigerate at least 1 hour, up to 48 hours.
Preheat the oven to 375℉.
Pour the reserved ¼ cup of sugar into a small bowl.
Roll the cookie dough into tablespoon sized balls.
Roll each cookie dough ball in the sugar until completely coated.
Place the cookies on a large baking sheet lined with parchment paper, spacing them 2½ inches apart.
Bake for 12-14 minutes.
Allow to cool in the pan for 5 minutes, then transfer to a wire cooling rack to cool completely.