Preheat the oven to 350℉ degrees
In the bowl of a stand mixer, cream butter, sourdough discard, powdered sugar, lemon zest, lemon juice, and vanilla extract on low/medium speed until light and fluffy.
In a separate bowl, whisk together the flour and salt, then add to your wet ingredients
Mix on low speed until just combined. If needed, use a rubber spatula to scrape the sides of the bowl to incorporate all the dry ingredients.
Your dough will be a bit crumbly after using a mixer, but begin to bring it all together with your hands once mixed, and you will no longer have crumbly dough.