In a small bowl, sprinkle the yeast over 1 cup of the warm water. Stir to combine and let sit until frothy, about 10 minutes.

In a stand mixer or large bowl, combine the flour, sugar, and salt. Slowly mix in the yeast mixture and butter. Add remaining water if needed to form a soft dough.

Knead the dough on a floured surface until smooth and elastic. Transfer to a bowl, cover, and let rise until doubled, 1-2 hours.

Preheat oven to 450˚F. Line a baking sheet with parchment or grease it.

Once risen, divide the dough in half and shape into ovals. Place on the baking sheet and cover; rest for 30 minutes.

In a pot, bring 8 cups of water and baking soda to a boil.

Gently place each loaf in boiling water for 30 seconds, turning halfway. Remove and place back on the baking sheet.

In a bowl, beat the egg yolk with water. Brush over each loaf and sprinkle with coarse salt. Cut slashes in the top.

Bake until deep brown and hollow sounding, about 15-20 minutes. Cool for 15 minutes before slicing.