Step 1

Whisk flour, kosher salt, and yeast in a large bowl until well combined. Gradually add 3½ cups warm water a few big splashes at a time, stirring with a wooden spoon to incorporate, until a thick batter forms. Continue to stir aggressively for a minute or so to get any lumps out. Cover bowl and chill at least 12 hours and up to 2 days.

Step 2

Grease an 18x13" sheet pan generously with 1 Tbsp. cold butter, making sure every corner and side is covered. Drizzle oil onto pan and spread with your hands until every surface is covered.

Step 3

Using a rubber spatula or plastic bench scraper, turn dough out onto sheet pan. Using your hands, gently spread dough in an even layer to edges of pan.

Step 4

Place baking soda in a small bowl or mug and slowly pour ½ cup boiling water over, stirring to combine with a pastry brush. (Heads up: It’ll get foamy!) Generously brush entire surface of dough with baking soda solution; set aside remaining baking soda solution. Let dough rise in a warm place until nearly doubled in size, bubbly, and loose-feeling, which can take anywhere between 30 minutes to over 1 hour, depending on the temperature of your kitchen and the season.

Step 5

Place a rack in center of oven; preheat to 475°. Gently brush dough with reserved baking soda solution (you may not use it all). Bake 10 minutes. Rotate pan from front to back and brush focaccia with 3 Tbsp. melted butter; sprinkle generously with pretzel salt. Continue to bake until top is deeply browned (like a pretzel!) and underside looks browned and crispy when pried up with a spatula, about 10 minutes longer.

Step 6

Let cool slightly. Using a fish spatula or other flexible metal spatula, release focaccia from pan and transfer to a wire rack. Let cool completely.