How should I change the recipe if I actually have a sourdough starter?

Honestly, I'm not super familiar with sourdough starters but I'm willing to give you my best guess! For this recipe, skip step one. In step four, replace the faux starter with 1 cup of your starter and cream it together with the butter and continue with the recipe. Since your starter is more mature and probably a little more flavorful, I'd decrease the vinegar to 1 teaspoon (instead of 2). The rye flour shouldn't make a huge difference here. The texture may be slightly different but I'm sure they'll still be just as delicious! If you try it out, l'd love to see how they turned out!

Is your sourdough starter a rye-based starter--do you feed it with all or part rye flour? If so, and if you have the right quantity (as specified in the recipe) to spare between feedings, jump ahead to step 2. Otherwise, add a little (25g?) of your sourdough starter to step 1 and reduce the dark rye flour and water slightly to compensate for your addition.