In a mixing bowl, combine softened unsalted butter, brown sugar, and granulated sugar. Beat until smooth and creamy, about 1-2 minutes.

Add the egg and vanilla extract to the butter mixture. Mix until fully incorporated and smooth.

Integrate the sourdough starter into the bowl, mixing well until fully combined.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the dark chocolate chips gently using a rubber spatula.

Cover the mixing bowl with plastic wrap and refrigerate the dough for at least 30 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Scoop tablespoon-sized portions of dough onto the prepared baking sheet, leaving space between each.

Bake for 10-12 minutes, until edges are set but centers may look slightly underbaked.

Remove from the oven and let sit on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.

Store any leftover cookies in an airtight container at room temperature for several days.

For longer storage, freeze cookies in a freezer bag or container for up to three months.

To reheat, microwave cookies for about 10-15 seconds.