Combine the yeast and 1 teaspoon of sugar in warm water in a mixing bowl. Set aside while the yeast dissolves, for about 10 minutes.
Using a stand mixer fitted with a dough hook, combine the flour, the rest of the sugar, and kosher salt.
Combine the yeast mix with the flour mix in the mixer at medium speed, until the dough is smooth.
Grease a mixing bowl and add the dough. Cover it with wrap or foil, and let it sit at a warm temperature until the dough rises. The dough should double in size in an hour.
Line two baking sheets and preheat the oven to 425 degrees.
On a lightly floured surface, divide the dough evenly into 16 pieces.
Roll the pieces into a 24-inch-long rope, and twist each rope into the classical pretzel shape.
In a heat-proof bowl, combine the boiling water with baking soda.
Stir until the baking soda dissolves.
Dip each piece of pretzel into the now-dissolved soda mixture, and sprinkle with flake salt.
Place the pieces on the lined baking sheets.
Try to leave 1 ½ inches of space between the pieces.
Bake until the pretzels turn golden brown, about 10 minutes.
Brush the pretzels with melted butter.