Warm the milk: Heat the milk until it’s warm to the touch, about 110°F/43°C. It should feel like a warm bath, not hot. Too hot will kill the yeast.

Combine wet ingredients: In a large bowl (or the bowl of a stand mixer), whisk the warm milk, sugar, melted butter, whole egg, and egg yolk until smooth.

Add dry ingredients: Stir in 3 1/2 cups of flour and the kosher salt. Mix with a wooden spoon or the dough hook until a shaggy dough forms. If the dough is very sticky, add more flour 1 tablespoon at a time.

Knead the dough: Knead by hand on a lightly floured surface for 8–10 minutes, or in a stand mixer on medium-low for 5–7 minutes. The dough should be smooth, springy, and slightly tacky but not gluey.

First rise: Lightly oil a bowl, place the dough inside, and turn to coat. Cover and let rise in a warm spot until doubled, about 60–90 minutes. If your kitchen is cool, it may take longer.

Prepare the pan and butter: Grease a 9x13-inch baking pan. Melt 2 tablespoons of butter for brushing.

Shape the rolls: Turn the dough out onto a lightly floured surface. Pat or roll into a rectangle about 12x16 inches and 1/2 inch thick. Using a bench scraper or knife, cut into 16 equal rectangles.

Form the Parker House fold: Brush the surface lightly with melted butter. Fold each rectangle almost in half (like closing a book with the top edge offset slightly), then press gently to seal the folded edge. This creates the signature layered look.

Pan the rolls: Arrange the folded rolls in the prepared pan, seam side down, snug but not crammed. Leave a little space between them to allow for expansion.

Second rise: Cover and let rise until puffy and touching, about 35–50 minutes. They should look airy and slightly jiggly when the pan moves.

Preheat the oven: While the rolls rise, preheat to 350°F (175°C). Place an oven rack in the center.

Bake: Brush the tops with more melted butter. Bake for 18–22 minutes, until the rolls are lightly golden on top and reach an internal temperature of about 190°F (88°C).

Finish with butter: Immediately brush the hot rolls with more melted butter. Sprinkle with flaky sea salt if you like. Cool in the pan for 10 minutes before serving.