Make the dough: add the warmed milk to the bowl of a stand mixer and whisk in the yeast. Leave it for a few minutes to bloom. Add the sugar, flour, salt, and potato flour to the bowl and affix the dough hook attachment. Knead the dough on low until it starts to come together around the dough hook, then add the butter and knead. If the dough looks a bit dry, add a tablespoon or two of water. Continue kneading until the dough once again comes around the dough hook and feels tacky. Set the dough on a countertop and knead by hand until smooth, just a minute or two. Transfer the dough to an oiled bowl and leave to rise at room temperature for about an hour, then transfer to the fridge for an overnight rest.
Prep the butter block: use a half sheet of parchment paper (9x13 inches) folded in half. Slice the butter horizontally into three pieces so you have three long rectangles and arrange them into the bottom half of the parchment paper. Fold the top half of the paper over the butter and use a rolling pin to press and roll the butter to (mostly) fill the parchment. Set in the fridge to chill and firm.
Laminate: After a long rest and chill, the dough should be very pliable. Take the butter block out of the fridge for 5 minutes, just to warm very slightly. Flour the counter well and turn the dough onto it. Flour the top as well (this is a sticky, thin dough). Roll out the dough so it’s just over twice the size of the butter block. Set the butter on the bottom half of the dough and fold the top half of the dough over it. Press the edges of the dough (the three sides) to seal in the butter.
Roll the dough out to twice its size and fold it in half. Turn the dough so the shorter side is now facing you and roll it out again, this time into a very long rectangle - flour the dough often with a thin layer so it doesn't stick, and avoid pushing the dough so far that it starts to resist. Aim for about 30 inches long and 10-12 inches wide. Now we’ll do a letter fold: fold the bottom third up then the top third over. Sprinkle with flour and roll the dough out again so you can perform one more, smaller, letter fold. The dough will start to resist now so it’s time for it to rest: wrap it in plastic wrap and set in the fridge for at least an hour (up to two).
Shape the loaf: Let the dough sit at room temperature for 10 minutes. Rub a standard size (9x5 inch) metal loaf pan with oil. On a well floured countertop, roll the dough so it’s about 9 inches wide and as long as you can get it (without too much resistance), it should be around 30 inches or so. Roll up the dough so you have a 9 inch log and place it, seam side down, in the prepared loaf pan. Cover with a damp towel and leave to rise at room temperature.
Bake the loaf: When you can press the top of the dough and it does not immediately spring back, it’s ready to bake. Preheat the oven to 350 F. Whisk together the egg yolk and milk then use a pastry brush to brush the top of the loaf with the egg wash. Set a cookie sheet under the loaf in the oven to catch any leaking butter. Bake the loaf until it’s deeply golden, about 40 minutes. Let cool in the pan. Sometimes the sides will stick to the pan, shimmy an offset spatula between the baked loaf and the pan to loosen it. The bread will be crispy the day it’s baked and will gradually soften. Store leftovers in an airtight container or freeze.