Preheat oven to 350 degrees F. In a mixer bowl combine butter, sweetener, and egg. Mix well for 1 minute. Add vanilla and sourdough starter. Mix well. Add cocoa powder, rye flour, salt, and baking soda. Mix well. Stir in mint chocolate chips. With a cookie scoop, scoop dough into parchment lined cookies sheets. Bake for 7-8 minutes. 24 cookies