Step 1

Lightly coat an 8½x4½” loaf pan with nonstick spray. Place ¾ cup (160 g) cold water in the bowl of a stand mixer; sprinkle with yeast and let sit until creamy, about 5 minutes.

Step 2

Meanwhile, whisk potato flour and 3½ cups (500 g) all-purpose flour in a medium bowl to remove lumps. Add flours and buttermilk to yeast mixture and mix on low speed with dough hook, increasing speed to medium as dough stiffens and comes together, about 5 minutes.

Step 3

Turn out dough onto a clean surface and knead until a smooth ball forms. Transfer to a large bowl and let sit 15 minutes to relax.

Step 4

Pull apart dough into several pieces and return to stand-mixer bowl. Add vinegar and salt and mix on low speed with dough hook until dough is smooth, stretchy, and no longer sticky, 5–8 minutes.

Step 5

With motor running, add butter 2 pieces at a time, scraping down sides of bowl as needed and mixing until incorporated before adding more, about 4 minutes between additions. Increase speed to medium and mix until dough starts to come away from sides of bowl and forms a ball, about 5 minutes longer.

Step 6

Transfer dough to a lightly floured bowl and cover with plastic wrap. Let sit at room temperature 30 minutes to relax.

Step 7

Turn out dough onto a lightly floured surface. Stretch an edge up and over to the opposite side of dough. Repeat 3 times, working your way around dough. Flip dough over, return to bowl, and cover with plastic wrap. Chill until doubled in size, 18–24 hours.

Step 8

Turn out dough onto a lightly floured surface and gently pat out larger air bubbles with floured hands. Fold sides of dough inward to make a 7”-long rectangle. Pat and tuck dough into a rough cylinder and place, seam side down, into prepared pan. (Make sure seam is dead center; it will help keep finished loaf from being lopsided and prevent it from splitting.)

Step 9

Let dough rise until outside edges have risen just above pan, 4–6 hours.

Step 10

Heat oven to 500°. Brush dough with milk. Using a wooden skewer or toothpick, very gently prick loaf 8–10 times evenly across surface, popping any larger bubbles. Place pan in oven and reduce oven temperature to 425°. Bake until bread is deep golden brown, 50–60 minutes. Remove pan from oven, gently ease bread from pan and return to oven, placing directly on rack. Bake until sides are firm but not hard, 10–15 minutes longer.

Step 11

Transfer bread to a wire rack and let cool completely before slicing (this may prove impossible, but try).

Step 12

DO AHEAD: Bread can be baked 4 days ahead. Store tightly wrapped at room temperature.