Make the dough: In a big bowl, mix the yeast, sugar and 1 1/3 cups warm water and let it sit until the yeast blooms, about 7 minutes. Add the melted butter, kosher salt, egg yolks, bread flour and all-purpose flour and knead the dough until smooth, 15 to 20 minutes.
Brush the bowl and the dough with olive oil, set the dough in the bowl and cover tightly with plastic wrap. Let rise on top of the fridge or in any warm, dry place for 30 to 40 minutes, or until doubled in size.
Make the pretzels: In a medium saucepan, mix the baking soda and 4 cups warm water until it is milky, then bring to a simmer over low heat. Preheat the oven to 425 degrees F. Cover 3 baking sheets with parchment and spray with cooking spray.
Punch down the dough. Cut into 12 to 16 pieces and let rest for 5 minutes. Roll each piece into a 2-foot-long rope. Lift the ends of the rope to make a "U," then cross one end over the other twice to make a double twist in the middle. Fold down the two ends and attach them to the bottom of the ?U? to form a pretzel shape.
Working in batches, dip the pretzels into the simmering liquid for 30 seconds, flipping once. Remove using a spider or two wooden spoons and shake off any excess liquid; place on a prepared baking sheet. Lightly flour the pretzels, cover loosely in plastic wrap and let rise for 20 minutes.
Brush the pretzels with the melted butter and use cooking spray to coat any little corners you can?t reach with butter. Sprinkle with the pretzel salt. Bake until browned, 8 to 12 minutes. Let cool for a few minutes and serve with mustard.