In the bowl of a stand mixer fitted with the paddle attachment, combine sourdough starter, water, and milk.
Mix on medium speed until smooth, about 1 minute.
With mixer running, add flour, yeast, salt, and butter.
When loosely combined, change to dough hook.
Knead on medium high until it forms a smooth dough, about 12-14 minutes.
Place dough in a covered container and allow to proof for 30 minutes.
Scale dough to 3 ounce pieces.
Form each piece of dough into a ball and place on a sheet of parchment paper.
Cover and let rest 20-30 minutes.
While dough is resting, line 2-3 sheet pans with parchment paper. Set aside.
Working with one piece of dough at a time, roll each into a rope 24-30 inches long.
Move dough into a U shape.
Pull ends of dough up, twist twice, and press ends down onto top of U.
Place on prepared pan.
Cover and let rise for 90 minutes.
While dough is rising, preheat oven to 425 degrees F.
The last 10 minutes of rising time, bring water to a boil in a large, wide pot.
Add baking soda. Be careful because it will boil up.
Slide dough into boiling water.
Let cook 1 minute.
Flip and cook for 1 more minute.
Using a spider or a slotted spoon, remove pretzels to parchment lined pans.
Sprinkle each pretzel with desired amount of pretzel salt.
Bake for 10-12 minutes, or until golden brown.