1. Prepare the Crust:

1. In a large bowl, combine warm water, yeast, and sugar. Let sit for 5-10 minutes until foamy. Stir in melted butter. In a separate bowl, whisk together flour, 1 tsp salt, and chopped rosemary. Gradually add the flour mixture to the yeast mixture, stirring until a dough forms.
2. Knead the dough on a floured surface for 5-7 minutes until smooth and elastic. Place in a greased bowl, cover, and let rise in a warm place for 1 hour or until doubled in size.
3. Preheat oven to 450°F (230°C). Line two baking sheets with parchment paper. Bring a large pot of water to a gentle boil and add the baking soda. Punch down the dough and divide into 12 equal pieces. Flatten each piece, place a mozzarella cube in the center, and pinch the dough tightly around it to seal completely.
4. Carefully drop 2-3 stuffed dough balls into the boiling water for 30 seconds per side. Remove with a slotted spoon and place on the prepared baking sheets. Brush each pretzel with the egg wash and sprinkle generously with coarse sea salt.
5. Bake for 12-15 minutes, or until deep golden brown. Let cool on the pan for 5 minutes before serving warm. The cheese will be very hot.