Make the dough:
Melt the butter in a small pot, add the water and stir to combine. Cook for a couple of seconds then take off the heat. Add the sugar. Check the temperature of the water with a thermometer, it should be 100-110°F (38-43°C). The water can’t be too hot or the yeast won’t work. When water has the right temperature, stir in the yeast. You can let stand the yeast mixture for 5 minutes to foam (a good way to check if your yeast is working, but it's optional).
In a large mixing bowl, combine flour and salt. (Adding the flour: measuring the flour with measuring cups is inaccurate so if that's the way you're measuring your flour, add 3 cups of flour first and then add the remaining 1/4 cup while kneading the dough if it’s still sticky. If using a digital scale you can add all the flour as written in one go).
Add the yeast mixture. Stir with a spoon until roughly combined.
Knead the dough by hand or with a stand mixer fitted with a dough hook attachment – about 3-4 minutes until the dough is smooth and not sticky. If it's too stickly just add a little bit more flour.
Cover the bowl with a kitchen towel and let stand in a warm place until roughly doubled in volume. You can put it in a turned off oven with a light on. It should take about 45-60 minutes.
Preheat the oven to 425°F (220°C).
Punch the dough to deflate it. Place the dough on a lightly floured counter or on a rolling mat, shape it into a thick disc and cut into 8 parts.
Shape the pretzels: Take one piece of the dough and roll it into a 1-inch (2.5 cm) thick and 23 inches (60 cm) long rope.
Form a U-shape with each end of the rope then holding the ends of the rope, cross them over then twist the ends together and pinch them to the sides to form a pretzel shape (see photos in the body of the post for a reference).
Place on a baking sheet lined with parchment paper and repeat with the remaining dough.
Cook pretzels in soda bath: Bring 5 cups of water in a medium/large pot to a boil. The pot can't be too big or you'd have to use more water to cover the pretzels (double water and baking soda accordingly) but it also can't be too small - the pretzels must be submerged in the water and have enough place to 'swim'.
When the water is boiling, add baking soda - do this carefully! it will foam vigorously. Boil 1 pretzel at a time, for 20-30 seconds (don't exceed this time), then remove with a slotted spoon back to the baking sheet. Sprinkle immediately with coarse / flaky salt. Repeat with the remaining pretzels.
Bake until golden, about 11 minutes.