Add the flour and salt into a large bowl, mix together.
In a small saucepan, warm the water with butter until they are very hot, but not boiling (temperature should be around 80-90 °C / 176-194 °F, that is when the water starts to move and steam).
Pour hot water with butter into the bowl with flour, mix with a wooden spoon until roughly combined.
Knead the dough using your hands or with a stand mixer fitted with a hook attachment, for about 5 minutes. A food processor can also be used (fitted with the dough blade). The dough should be smooth, soft and elastic, it shouldn’t stick to your hands. When you follow the recipe (especially if you weight the ingredients, instead of using measuring cups), the dough should be perfect, but if for some reason it’s not, add some water if it’s too dry, or a little bit flour if it’s too wet. If the dough already forms a ball, is relatively smooth, but still tough, it means that it’s not ready, it must be further kneaded until soft and elastic (after resting time it will be even softer).
Wrap the kneaded dough in plastic foil, leave to rest for about 30 minutes.