Make the Sponge: In a stand mixer bowl, combine warm water, yeast, and flour on low speed. Cover and let rise for about 45 minutes until puffy with small bubbles.

Mix the Dough: Add eggs and egg yolk one at a time, mixing in sugar, salt, and 1 1/2 cups of flour. Switch to a dough hook, gradually adding remaining flour until incorporated. Knead on medium speed for about 10 minutes.

Add the Butter: With the mixer running, add butter one tablespoon at a time until absorbed, about 4 minutes.

First Rise: Shape the dough into a ball, cover, and refrigerate for 8 to 12 hours.

Shape the Loaf: Spray a 9x5-inch loaf pan with cooking spray. Roll out the dough into a rectangle, fold it, and place it in the pan seam side down.

Second Rise: Cover and let rise until puffy, about 2 hours.

Bake the Loaf: Preheat the oven to 400°F. Brush with egg wash, reduce temperature to 375°F, and bake for 35 to 40 minutes until golden brown and a thermometer reads 190°F to 200°F.

Cool: Let cool on a rack before slicing. Store wrapped for up to 24 hours or toast leftover slices.