Let ½ stick unsalted butter sit at room temperature until softened, about 45 minutes.

Place 3 cups bread flour, ½ cup instant mashed potato flakes, ¼ cup granulated sugar, 1 tablespoon instant yeast, and 1 tablespoon kosher salt in the bowl of a stand mixer fitted with a dough hook. Whisk with a fork to combine.

Add 1 cup water and 1 large egg. Mix the dough on low speed until it is soft, smooth, and starts to come away from the sides of the bowl, 5 to 7 minutes. Add the butter and continue to mix on low speed until butter is fully incorporated, 4 to 5 minutes. The dough will likely stick to the sides of the bowl. Use a rubber spatula or a plastic bowl scraper to scrape down the sides of the bowl.

Cover with a plate or lid. Let the dough rest until puffy and risen, 45 to 90 minutes. It may come close to doubling in size but will be OK if doesn’t. (If your room is warm, the dough will rise faster; in a cool room, the dough will take longer.)

Lightly grease a 8 ½x4 ½-inch metal loaf pan with cooking spray. Alternatively, lightly brush with oil or softened butter. Line the pan with parchment paper, letting the excess hang over the long sides to form a sling.

Lightly dust a work surface with flour. Turn the dough out onto the floured counter and, using your hands, pat it into a roughly rectangular shape. The size and thickness don’t really matter. The dough may be soft and even slightly tacky but should not be so sticky that it is hard to work with. Roll the dough into a log about the size of the pan and transfer to the prepared pan. Cover the pan loosely with lightly oiled plastic wrap and let it rest until it has risen about 1 inch above the surface of the pan, 60 to 90 minutes.

Arrange a rack in the center of the oven and heat the oven to 350°F. Uncover the bread and bake until it is deep golden-brown on top, 25 to 45 minutes. An instant-read thermometer inserted into the center should register 200°F.

Remove the bread from the oven and let it rest in the pan for 10 minutes before turning out onto a wire rack to cool completely. Let the bread cool completely before slicing.