Step 1

Arrange a rack in the middle of the oven and heat the oven to 400°F. Place 1 stick unsalted butter in a medium microwave-safe bowl and microwave on HIGH power in 10-second bursts until fully melted, 45 to 60 seconds total. (Alternatively, melt on the stovetop over low heat.) Pour the melted butter into a 8x8-inch baking dish and swirl it around so it coats the sides.

Step 2

Place 2 1/2 cups all-purpose flour, 1 tablespoon baking powder, 4 teaspoons granulated sugar, 1 1/2 teaspoons kosher salt, and 1 teaspoon baking soda in a large bowl and whisk to combine. Add 1 1/2 cups cold buttermilk and stir with a rubber spatula until a shaggy, sticky dough forms and there are no pockets of dry flour left.

Step 3

Transfer the dough into the prepared baking pan. Using damp hands, press the dough evenly into the pan until it touches all the sides. (The dough will be swimming in melted butter.) Cut the dough all the way down to the bottom of the pan in a 3x3 grid to make 9 squares. (The squares will still be touching each other.)

Step 4

Bake until the top is golden-brown and the biscuits have puffed up significantly, 25 to 27 minutes. Immediately cut along the scores again to separate the biscuits. Let cool for at least 15 minutes in the pan before serving.