In a large mixing bowl, dissolve the sugar in warm water, then sprinkle the yeast on top. Let it rest for about 5-10 minutes until it becomes frothy.

Add the salt and flour, one cup at a time, mixing until you form a soft dough. You may need to adjust the flour slightly, depending on the humidity and absorbability.

Knead the dough on a floured surface for about 5-7 minutes or until it becomes smooth and elastic.

Place the dough in a greased bowl, cover with a damp cloth, and let it rise in a warm area until it doubles in size (about 1 hour).

Preheat your oven to 450°F (230°C).

Once the dough has doubled, punch it down and divide it into 12 equal pieces.

Roll each piece into a long strip (about 24 inches).

Take each strip and form a 'U' shape, then twist the two ends together a few times before folding down the ends onto the bottom of the 'U' to create the pretzel shape.

To fill, make a small cut along the bottom of the twist, carefully insert a cube of mozzarella, and press the dough around it to seal.

In a large pot, bring about 8 cups of water to a boil, then add the baking soda (be careful; it may bubble up).

Gently place the pretzels into the boiling water, no more than two at a time. Boil each side for about 30 seconds, then use a slotted spoon to transfer them to a lined baking sheet.

Brush each pretzel with the beaten egg (if using) and sprinkle with sea salt.

Bake for 12-15 minutes, or until golden brown.

Once out of the oven, let them cool for a few minutes, and enjoy the melty mozzarella goodness inside with your favorite dipping sauce!