In a large saucepan, combine sugar, milk, salt, and cocoa powder. Stir the mixture on medium-high heat until all the ingredients are dissolved.

Bring the mixture to a rolling boil for 1 minute, stirring constantly to prevent burning.

Remove the saucepan from the heat. Quickly stir in the peanut butter, butter, and sourdough starter until fully incorporated.

Fold in the rolled oats until everything is well mixed.

Drop the cookie mixture onto a prepared baking sheet lined with parchment paper. Use a medium cookie scoop for uniform cookies.

Let the cookies set at room temperature until they harden, about 30 minutes to 2 hours.

Once hardened, store the cookies in an airtight container to keep them fresh.