In the bowl of the standing mixer, whisk together the water, yeast, salt, sugar, butter. Add the flour, and then blend at low speed until combined -- about 30 seconds. Increase speed to "medium," and knead until the dough cleans the bowl -- 3-5 minutes. Cover the bowl with plastic wrap, and let the dough rise in a warm location until doubled in volume -- about 90 minutes.
Deflate the dough and scrape it onto a non-floured work surface. Form the dough into a rough rectangle, and then cut it into 8 equal-size pieces. Form the pieces into rolls and place them on the prepared baking sheet. Cover loosely with plastic wrap, and let rise until nearly doubled in volume -- about 30 minutes.
Meanwhile, in the large pan or soup pot, bring 10 cups of water to a boil. Then reduce the heat so the water just simmers. Preheat the oven to 425°F.
A little at a time, add the 1/3 cup baking soda to the simmering water (the water will foam). Then place 2 or 3 rolls in the water bath, and let them poach for 30 seconds on each side. Transfer the rolls to the baking sheet. (Or, use a fresh parchment- or silicone mat-lined baking sheet. Poach the remaining rolls.
Brush the poached rolls with the beaten egg. Then, if desired, sprinkle the rolls with kosher salt -- just a pinch for each roll. Use a sharp knife to cut two 1-inch long, 1/4-inch deep slashes on top of the rolls.
Bake in the preheated oven until the rolls puff, the slashes split, and the tops brown attractively -- 15-20 minutes. Cool completely on a wire rack.