5 tablespoons unsalted butter
240 grams whole milk
200 grams active sourdough starter
1 egg
50 grams sugar
500 grams all purpose flour
13 grams salt
For finishing: 2 tablespoons unsalted butter (melted), kosher salt
Melt the butter in a large skillet over medium-low heat. Once melted, remove from heat and pour in the milk.
To the bowl of a stand mixer fitted with the paddle attachment, add the starter, egg, and sugar and mix until combined. Add the butter mixture and mix until combined. Add the flour and salt and mix until no dry flour remains, scraping down the sides of the bowl as needed. Switch to the dough hook and mix on LOW speed for 8-10 minutes, until the dough begins to look smooth and passes the windowpane test (you can pull a piece of dough apart enough to let light pass through it without the dough tearing). Cover and let rise until doubled in size - 12 to 14 hours.
Grease a 9 by 13-inch baking dish. Turn dough out onto a lightly floured surface. Divide into 15 pieces and shape into rolls. Arrange in a 3 by 5 grid in the baking dish. Cover and allow to rise at room temperature until significantly puffed up, about 1 ½ - 3 hours.
Preheat oven to 350 degrees. Bake rolls on middle rack for 25 - 30 minutes, until browned to your liking and baked through. Remove from oven, brush with the melted butter, and sprinkle with salt.
Fully cooled leftovers will keep in an airtight container or ziplock bag at room temperature for up to 3 days. Reheat in the microwave wrapped in a slightly damp paper towel for optimal softness.