They are soft, have chewy centers, crisp edges, and taste amazing. It's one of my favorite ways to use up sourdough discard. Made only with egg yolk, I think you can imagine how rich and delicious these sourdough discard cookies are!
In a large mixing bowl, using an electric mixer fitted with a paddle attachment, cream the butter, sugar, and brown sugar on medium-high speed for 2-3 minutes.
Add the egg yolk and vanilla and mix for about 1 minute, until well combined and creamy. Mix in the sourdough starter for about 1 minute until combined.
Then add the flour and salt and mix on low speed just until incorporated, about 30-60 seconds. Fold in the chocolate until it is well distributed.
Cover and chill for 2 hours or up to 2 days.*
Line two baking sheets with parchment paper and preheat the oven to 350°F (177°C).
Scoop 12 equal-sized balls (about 2 tablespoons [45g]) of cookie dough, roll into even balls, and place them, 4 inches (10 cm) apart, on the prepared baking sheets.
Bake one sheet at a time for 10-12 minutes, or until the edges are set and lightly browned and the centers look pale, puffy, and dry, with tiny cracks all over the surface.
Remove from the oven and let cool on the baking sheet for about 5 minutes, or until firm enough to move. Then, transfer to a wire rack and let cool completely. The cookies will stay fresh in an airtight container at room temperature for up to 4 days or freeze for up to 3 months.
Notes
* If you're chilling the dough for more than 2 hours, you may want to let it sit at room temperature for 15-30 minutes before rolling it until it's a soft and spoonable dough.