In the bowl of a stand mixer fitted with the hook attachment, add the warm water, brown sugar, and active dry yeast. Give a quick mix and let bloom for 5 to 8 minutes, until very foamy.
Add the sourdough discard and salt. Mix just to combine.
Add 3 cups of flour. Using the hook attachment on LOW speed, mix until all liquids are absorbed by the flour.
Add the cubed butter. Increase speed to Medium-Low to knead until a smooth and pliable dough, about 5 to 8 minutes. Add up to 1/2 cup more flour as needed.
Transfer to a greased bowl, cover tightly with plastic wrap and set aside in a warm environment for 1 to 2 hours, until doubled in volume.
On a clean surface, divide the risen dough into 12 pieces. Roll each into a tight ball. Place on a parchment paper lined baking pan.
Cover with plastic wrap to rise in a warm environment for 30 minutes, until doubled in size.
In the meantime, preheat oven to 425F.
Working one dough ball at a time, use a small cup to press down the centers (so you have a 1/2" border) then gently stretch out to widen the centers. TIP: You don't want the dough in the center to be see-through or super thin, or else it won't hold the filling well.
Bring a large pot of water to a boil. Stir in the baking soda.
Working 2 to 3 at a time, add into the boiling water face-side up. Boil for 10 to 20 seconds on each side, no longer than 30 seconds or they can develop a bitter taste, then place face-side up back on the baking pan. Use a slotted spoon and spatula to pick up face-side down, then flip face-side up onto the baking pan so no water carries over.
While warm, sprinkle the edges with coarse salt.
Bake on the middle oven rack for 18 to 22 minutes, rotating halfway through, until deeply golden brown.