In a large mixing bowl, combine warm water, active dry yeast, and granulated sugar. Let sit for about 5 minutes until it foams gently.

Gradually mix in all-purpose flour and salt, kneading the dough for 5-7 minutes until smooth and elastic. Cover and let rise for 45 minutes until doubled.

Punch down the dough, divide into 12 pieces, and roll into long ropes about 24 inches long. Form into pretzel shapes and place on a lined baking sheet.

Boil 10 cups of water, stir in baking soda until dissolved, and drop each pretzel into the boiling solution for 30 seconds. Remove with a slotted spoon.

Preheat oven to 425°F (220°C). Place mozzarella in the center of each pretzel, pinch dough closed around the cheese to seal.

Brush each pretzel with melted butter, sprinkle with coarse sea salt, and bake for 20 minutes until golden brown.

Cool on a wire rack for a few minutes before serving warm with mustard or marinara sauce.