Ensure butter is softened to room temperature and gather all ingredients.

In a large mixing bowl, cream butter, granulated sugar, and brown sugar on medium speed for 2 to 3 minutes until pale and fluffy.

Add the egg and vanilla extract and beat on medium speed for 1 minute until fully incorporated.

Fold in the sourdough starter discard gently with a spatula until just combined.

In a medium mixing bowl, whisk together all-purpose flour, baking soda, and salt until evenly distributed.

Pour the dry ingredients into the wet mixture and fold gently until no streaks of flour remain.

Fold in the chocolate chips or other mix-ins until evenly distributed.

Preheat the oven to 375°F (190°C). Line baking sheets with parchment paper and scoop dough by 1.5 tablespoon portions, spacing 2 inches apart. Bake for 12 to 14 minutes until edges are golden and centers are slightly underbaked.

Let cookies rest on the baking sheet for 2 minutes before transferring to a cooling rack to cool completely.