PREP: Preheat oven to 425 degrees F. Line a large sheet pan with parchment paper or a Silpat liner OR line a 5-quart cast-iron pot with parchment paper. Set aside. Melt butter in a medium-sized microwave-safe bowl.
LIQUIDS: Take the butter from the microwave and pour in 1 cup (237g) cold buttermilk. Stir with a fork and set aside for about 5 minutes. In another medium-sized bowl add the remaining 3/4 cup (176g) buttermilk and the large egg. Whisk together until smooth and set aside.
DRYINGREDIENTS: Add 4-1/2 cups, 3 tablespoons sugar, 2-1/2 teaspoons baking powder, 1 teaspoon baking soda, and 1-1/2 teaspoons salt to a very large bowl. Whisk together until combined. Make a well in the middle of the bowl and pour in the buttermilk + egg mixture. Stir the butter and buttermilk mixture again, mixing until small clumps form (we want this mixture as clumpy as possible!). Use a spatula to scrape every bit of this mixture on top.
MIX: Use a strong spatula or wooden spoon to mix ingredients until just combined and no streaks of flour remain. Take care not to overmix or overwork the dough. Sprinkle 2 tablespoons extra flour onto a clean work surface. Scrape out all of the dough right on top and sprinkle with 2 more tablespoons of flour. Gently knead the bread about 6-8 times to bring everything together into a wide and smooth ball.
BAKE: Gently transfer the ball to the prepared baking sheet or pot and pat it down to about 1-1/2 inches in height. Use a sharp serrated knife to cut an "X" across the surface, about 1/4-inch deep. Bake for 25 minutes. After 25 minutes, do not open the oven, but reduce the heat to 400 degrees F and bake for another 20-30 minutes or until the base sounds hollow when tapped in the middle (25 mins in my oven). See Note 3.
COOL: Remove tray from oven and let stand for 10 minutes before carefully transferring to a wire cooling rack. Let the bread cool for at least 30 minutes before slicing into it (the steam is still cooking the bread!) -- Note 4.
Note 1: Buttermilk: While it would seem like any milk will work here, we need the acidity from the buttermilk to counteract the baking soda and powder to create the rise in the bread. Real buttermilk is a must!Note 2: Flour: When measuring flour for this bread, get an exact weight using a food scale or spoon and level the flour. If you press a measuring cup into a container and push the flour into the cup, you’ll end up with way too much flour. Here’s how to spoon and level flour.Note 3: Tapping the bread: To test for doneness, tap the bottom of the loaf and listen for a hollow sound. To do this, wearing oven mitts, tip the pan so that the bread comes out of the pan and onto your hand or a cutting board. Lift the bread with your hand and tap with your finger. Tapping while the bread is on a counter or still in the pan will not give you an accurate indication.Note 4: Cooling Time: Once you pull it from the oven, allow time for the bread to stand on the sheet pan for about 5-10 minutes and then on a wire cooling rack for at least 30 minutes -- this rest time is just as important as the bake time! The residual heat and steam ensure the inside is fully baked through. As tempting as it is to cut in, don't rush it because you'll likely end up with a slightly doughy center.