In a large mixing bowl, whisk the cane sugar and brown sugar until combined. Add the butter and mix, either with electric beaters or by hand, until well incorporated and fluffy.

Add the starter discard to the bowl and mix to combine. Stir in the egg and mix until smooth. If the dough looks like it is breaking or curdling at this point, stop mixing, but note that it will come back together in step 3.

Sift in the flour, salt, and baking soda. Use a wooden spoon to stir until just mixed and no streaks of flour can be seen in the dough.

Add the chocolate and gently fold into the dough until well incorporated.

Cover the bowl with a plate and place into the refrigerator. Refrigerate for a minimum of 2 hours, or up to 24 hours.

Once the dough has rested, it's ready to bake. Preheat the oven to 375ºF / 190ºC* and line two large baking sheets with parchment paper.

Scoop balls of dough about 2 tablespoons in size onto the prepared baking sheets. There should be about 20 cookies total.

Bake the cookies, one tray at a time (see note) for 10-12 minutes, or until lightly golden and the outer edge feels just firm to touch.

Cool for 10 minutes on the tray before removing and cooling fully on a rack. The cookies will keep for about a week in a sealed container at room temperature.