Heat milk in a Butter Warmer or Saucepan over medium until just warm (110–115F).

Pour warm milk into a large bowl if mixing by hand, or the bowl of a stand mixer. Add ¼ cup warm water, sugar, yeast, and salt and whisk together.

Cut 4 Tbsp. room temperature butter into 8 pieces and add to milk mixture. Stir, breaking up the butter pieces and let stand until foamy and puffed, about 10 minutes.

Lightly beat 1 egg and stir into milk mixture.

Add the flour in three additions, stirring with a Wooden Spoon, Spatula, or using the stand mixer paddle on low speed to incorporate each addition before adding the next.

Vigorously mix by hand or with the stand mixer on medium until a rough mass of dough forms; it will be quite shaggy and sticky.

Transfer dough to a lightly floured surface and use floured hands to knead dough until smooth, 7–9 minutes, or use the stand mixer dough hook, 5 minutes. At this point the dough should be smooth, elastic, and a little tacky.

Form dough into a ball and transfer to a lightly oiled bowl; turn to coat. Cover loosely with plastic wrap. Let stand in a warm place until doubled in size, 20–30 minutes.

While your dough is rising, set a large Enameled Cast Iron Skillet over medium-low.

Melt 2 Tbsp. butter and swirl to coat bottom and sides. Remove from heat and set aside to cool.

Transfer dough to a floured work surface. Use a Chef Knife or bench scraper to cut dough into 12 pieces.

Shape each piece into a smooth ball by rolling on the surface. Place smoothest side of each dough ball facing up in the buttered and cooled Skillet. Cover with plastic wrap again and let rolls rise until large and puffed, 20–30 minutes.

Preheat oven to 350F and place rack in center of oven.

Brush rolls with egg wash and sprinkle with flaky salt. Transfer to oven and bake until puffed and golden brown, 25–35 minutes.

Heat remaining 2 Tbsp. butter in a Butter Warmer or Saucepan over medium until melted.

Allow rolls to cool slightly, then brush with melted butter and serve.

Serve rolls warm or at room temperature. Rolls will keep in an airtight container at room temperature for 2–3 days or frozen for up to 3 months.

To make the rolls in advance of baking, form rolls in Skillet, cover with plastic wrap, and refrigerate overnight. When ready to bake the next day, allow to come to room temperature and rise before baking (they will take a little longer to rise if refrigerated, about 60 minutes). To freeze, allow rolls to rise 10 minutes then transfer to a parchment-lined Sheet Pan. Wrap tightly and freeze for up to 1 month. Defrost rolls in refrigerator overnight before baking.