In the bowl of a stand mixer or a large mixing bowl, whisk together the warm water, sugar and active dry yeast. Let it sit for 5–10 minutes until frothy.
Add the melted butter, salt, and sourdough discard. Stir to combine. Add 4 cups of flour, mixing with a wooden spoon or dough hook attachment on low speed.
Knead on medium-low speed in the mixer for 5-8 minutes, or knead by hand on a lightly floured surface. If you’re using a mixer and the dough still feels too sticky after 5 minutes, add flour 1 tablespoon at a time until it comes together. If you’re kneading by hand and the dough becomes difficult to work with, sprinkle in flour 1 teaspoon at a time to help manage stickiness. If your dough is too dry add 1 tsp of water at a time. The dough should feel soft, smooth, and slightly tacky, when you press it with your finger, it should gently bounce back.
Transfer the dough to a greased large bowl, cover with plastic wrap or a clean kitchen towel, and let it rise in a warm place for 1 hour, or until doubled in size.
Punch down the dough and turn it out onto a lightly floured surface. Divide the dough into 12 equal pieces. Roll each piece into a smooth, tight ball by tucking the edges under and sealing the bottom. Gently press down on each bun to slightly flatten, this helps them spread just a bit for that classic pretzel bun shape.
Preheat your oven to 400°F (204°C). Line a baking sheet with parchment paper. Set it aside for now.
In a large pot, bring 8 cups of water to a boil. Slowly stir in the baking soda. Working in batches, gently lower the buns into the water with a slotted spoon for 20–30 seconds each. Place them on a parchment-lined baking sheet.
Use a sharp knife or razor blade to score the tops. Sprinkle generously with pretzel salt or flaky sea salt.
Bake at 400°F (204°C) for 20 - 22 minutes, or until deeply golden brown.
Let cool on a wire rack for a few minutes. Serve warm or store in an airtight container at room temperature for up to 3 days.