Stir yeast with sugar until liquid. Knead with 550 g flour, butter, 2 teaspoons salt and approx. 250 ml lukewarm water with the dough hooks of the mixer for approx. 5 minutes. Cover and leave to rise in a warm place for about 1 hour.
Grate the mozzarella roughly. Roll out the dough on a floured work surface in a rectangle (approx. 50 x 50 cm). Halve the dough, cut the halves crosswise into 5 strips each. Cover lengthwise in the middle with 1-2 tbsp. cheese. Fold dough lengthwise. Roll into strings of about 50 cm length. Form into pretzels and place on two baking trays lined with baking paper. Cover and leave to rise for about 15 minutes.
Preheat oven (electric cooker: 200 °C/recirculating air: 180 °C/gas: see manufacturer). Wash rosemary, chop needles. Boil up approx. 1 l water, 1 teaspoon salt and baking soda in a small pot. Dip the pretzels one by one with the upper side into the boiling water and let them steep for about 30 seconds. Turn them over and let them steep for about 10 seconds. Lift out, drain well and put back on the baking paper. Sprinkle with rosemary and sea salt. Bake the pretzels tray by tray in a hot oven for 12-15 minutes. Let them cool down.