Step 1

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.

Step 2

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.

Step 3

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.

Step 4

Cover and chill for 2 hours or up to 2 days.*

Step 5

Line two baking sheets with parchment paper and preheat the oven to 350°F (177°C).

Step 6

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.

Step 7

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.

Step 8

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.