Heat the 1/2 cup of milk to 100-110 degrees. Mix with packet of yeast and let sit for ten minutes.
In base of a mixer fit with dough attachment, mix together buttermilk, flour, salt, sugar, 1/4 cup of softened butter, and egg. Once well mixed, stir in the yeast/milk mixture. Continue mixing until a ball of dough forms.
Spray a bowl with cooking spray and place dough in bowl, then flip dough so that both sides of dough are covered with spray. Cover bowl with a clean dish towel or plastic wrap. Let sit in a warm place, without a draft, and rise until doubled in size, about 75-90 minutes.
Gently pat down dough. Spread dough in a long rectangle on a working surface. Cut into 16 equal parts and form each chunk of dough into a ball.
Melt 2 of the remaining tbsp of butter. Brush the inside of a 12" cast iron skillet with some of the butter. Place dough in skillet, evenly spaced.
Brush the top of each dough ball with the remaining melted butter. Cover with a dish towel and allow bread to rise until almost doubled in size, about 60 minutes.
Preheat oven to 375 degrees. Bake rolls until golden on top, about 18-20 minutes. Remove from oven. Melt remaining tbsp of butter and brush over top of rolls. Lightly sprinkle the top of each roll with kosher salt, to taste.