Preheat the oven to 350 degrees.**(see note) Line two baking sheets with parchment paper and set aside. Mix the dry ingredients together using a whisk or dough hook and set aside.
Cream together the butter and both sugars and then add the vanilla and the starter and mix until just combined. Add dry ingredients to the wet ingredients and, again, mix until just combined. Stir in the chocolate chips with a wooden spoon or sturdy spatula.
Use the cookie scoop to place up to 8 cookies on each parchment-lined baking sheet. If you used room-temperature butter, chill the trays with the cookie dough balls for 30 minutes before baking and preheat oven when chill time is almost finished.***(see note)
Bake cookies for 18 minutes and swap and rotate trays halfway through bake time. Transfer each cookie sheet to a wire rack and lightly press the bottom of a drinking glass onto the top of each cookie to flatten slightly. Let cookies cool completely on the trays before serving or storing.
For best results, these sourdough discard chocolate chip cookies should be kept in an air-tight container at room temperature or frozen for up to three months.