In a large bowl, mix the sourdough starter, warm water, and sugar until combined.
Add the flour and salt, mixing until a dough forms.
Knead the dough on a floured surface for about 5-7 minutes until smooth.
Cover the dough with a damp cloth and let it rest for 1 hour.
Preheat your oven to 450°F (232°C) and line a baking sheet with parchment paper.
Divide the dough into 8 equal pieces and roll each into a long rope (about 24 inches).
Shape each rope into a pretzel form, twisting the ends together.
In a large pot, bring water to a boil and add baking soda.
Boil each pretzel for about 30 seconds, then remove and place on the baking sheet.
Brush each pretzel with melted butter and sprinkle with coarse salt.
Bake for 12-15 minutes until golden brown.
Let cool slightly before serving with your favorite dip.