In a small bowl, dissolve the yeast in warm milk and let it activate for 5 minutes. In a large mixing bowl, combine the activated yeast mixture, brown sugar, sourdough discard, bread flour, salt, and softened butter. Mix until a dough forms. Knead the dough on a floured surface for about 8-10 minutes until smooth and elastic. Place the dough in a lightly greased bowl, cover, and let it rise for 1 hour or until doubled in size. Preheat the oven to 220°C (425°F) and line a baking sheet with parchment paper. Divide the dough into small bite-sized pieces and roll each into a ball. Bring the hot water and baking soda to a gentle boil to create the baking soda bath. Carefully drop the dough bites into the baking soda bath for 30 seconds, then remove with a slotted spoon and place on the prepared baking sheet. Brush each pretzel bite with melted butter and sprinkle with pretzel salt. Bake for 12-15 minutes until golden brown. Brush again with melted butter right after baking for an extra buttery finish. Allow to cool slightly and serve warm.