Step 1

Put the warm water in a liquid measuring cup and sprinkle the yeast on top. Let stand until foamy, about 5 minutes. Meanwhile, in the bowl of a stand mixer fitted with the dough hook, combine the flour, dry milk, sugar and salt and beat on low speed until combined, about 1 minute. Pour the yeast mixture into the flour mixture. Add the whole eggs, egg yolk and butter and beat on medium-low speed until the dough is smooth and elastic, about 15 minutes.

Step 2

Transfer the dough to a lightly floured surface and knead gently for 1 minute, then form into a ball. Place the dough in a well-oiled bowl, turning to coat with the oil. Cover the bowl with a kitchen towel and let rise in a warm spot until doubled in size, about 1 1/2 hours. Remove the towel, cover the bowl with plastic wrap and refrigerate for at least 6 hours or up to 12 hours. Remove the bowl from the refrigerator and let the dough rise in a warm spot until doubled in size, about 2 hours.

Step 3

Lightly grease a brioche pan with oil. Transfer the dough to the prepared pan, gently reshaping it as needed to fit. Cover with a kitchen towel and let rise in a warm spot until the dough has risen to just above the edge of the pan, about 1 hour.

Step 4

Preheat an oven to 400°F (200°C).

Step 5

In a small bowl, lightly beat the remaining egg white. Brush the dough with the egg white and sprinkle with the rosemary leaves and flaky sea salt. Bake for 20 minutes, then reduce the oven temperature to 350°F (180°C). Continue baking until the bread is deep golden brown and an instant-read thermometer inserted into the center registers 165°F (74°C), 25 to 30 minutes more. If the bread begins to brown too quickly, cover with aluminum foil.

Step 6

Transfer the brioche pan to a wire rack and let cool for 10 minutes, then turn the brioche out onto the rack. Let cool another 10 minutes, then cut into slices and serve warm with butter. Serves 8.

Step 7

Williams Sonoma Test Kitchen