In a large bowl, whisk together flour, sugar, salt, and yeast. Add cool water and softened butter. Mix until a shaggy dough forms, then turn out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. The dough should be slightly tacky but not sticky.
Stand Mixer
Add flour, sugar, salt, yeast, and butter to the mixer bowl fitted with the dough hook. Pour in the cool water. Mix on low (speed 1-2) until it comes together, about 1-2 minutes. Increase to medium-low (speed 2-3) and knead for 6-8 minutes until the dough pulls cleanly from the sides of the bowl and looks smooth. If it climbs the hook, it's too dry - add a splash of water. If it won't come together, add flour a tablespoon at a time.
The dough is ready when you can do the windowpane test: pinch off a small piece and stretch it thin. If you can see light through it without it tearing, gluten is developed.
Overnight Rise (Cold Ferment)
Place dough in a lightly oiled bowl, cover tightly with plastic wrap, and refrigerate for 12-18 hours. The slow, cold ferment develops complex flavors and a better texture.
Bring to Room Temperature
Remove dough from fridge and let it sit at room temperature for 1-2 hours. It will warm up and become easier to shape.
Shape
Press down on the dough with your palm a few times to release the large gas bubbles. Fold the edges toward the center, flip seam-side down, then shape into one large loaf or two smaller loaves. Handle gently to preserve some of the smaller air bubbles. Place on a parchment-lined baking sheet.
Final Rise
Cover loosely and let rise at room temperature for 1-1.5 hours until puffy and the dough slowly springs back when poked.
Score and Prep
Using a sharp knife or lame, score the top with 3 diagonal slashes about 1/4 inch deep. Brush with water or milk for a softer crust.
Bake
Preheat oven to 375°F. Bake for 28-35 minutes until deep golden brown and the loaf sounds hollow when tapped on the bottom. The extra fermentation time means more sugars for browning.