Heat milk to between 105°F and 110°F. Stir in yeast and sugar. Let sit until frothy, 8 to 10 minutes (if it doesn’t froth, discard and start again). Whisk together flour and kosher salt in another bowl; make a well in the center. Add milk mixture and 3 tablespoons butter. Gently stir just until incorporated (the mixture will look shaggy). Transfer to a lightly floured work surface and knead until smooth, 1 to 2 minutes. Transfer to a lightly oiled bowl; cover with plastic wrap or kitchen towel. Set in a warm place and let rise until doubled in size, 45 minutes to 1 hour. Lightly oil a 9-inch round cake pan. Turn dough out onto a work surface and cut into 16 pieces. Roll pieces into balls and arrange in the prepared pan. Cover with plastic wrap or a kitchen towel and let rise until doubled in size, 25 to 35 minutes. Preheat oven to 350°F. Melt remaining 3 tablespoons butter in a small saucepan. Gently brush half over tops of rolls. Bake until puffed and lightly golden brown, 25 to 27 minutes. Brush with remaining melted butter and sprinkle with flaked salt. Let cool 5 minutes before removing from the pan.