Whisk flour, yeast, sugar, and salt in a mixer bowl. Add warm milk and egg, mix to a shaggy dough. Add soft butter gradually and mix until absorbed.

Knead on medium speed for 8 to 10 minutes until the dough is smooth and passes the windowpane test. Place in a greased bowl, cover, and rise until doubled, about 1 to 1 1/2 hours.

Punch down dough and divide into 8 pieces. Flatten each piece into a disk, place a cold butter cube in the center with a pinch of flaky salt, seal tightly, and roll into a smooth ball. Place seam side down on a parchment lined sheet.

Cover and let rise until puffed and almost doubled, about 40 to 50 minutes. Preheat oven to 350°F (175°C). Brush rolls with egg yolk and milk wash, then top with sesame seeds if using.

Bake for 20 to 22 minutes until deeply golden. Sprinkle flaky salt on top immediately after baking. Cool 5 minutes on the pan, then enjoy warm.