Preheat oven to 350°F/177°C. Line a baking sheet with parchment paper (optional).
In a medium/large bowl, cream together the butter and brown sugar until very well combined (2-3 minutes). Mix in the sourdough discard followed by the egg yolk and vanilla extract.
In a small bowl, whisk together the flour, baking soda and salt. Stir the dry ingredients into the wet ingredients just until the flour streaks are gone. Mix in the chocolate chips.
Scoop cookie dough balls (about 2 tablespoons each) onto a baking sheet 2-3 inches apart. Top with extra chocolate chips (optional).
Bake on the middle rack until the cookie edges are lightly golden (8-11 minutes).
Let the cookies cool on the baking sheet until they're firm enough to transfer to a cooling rack (5-10 minutes).
