Combine flour and rosemary. Cut 1/3 cup butter into flour mixture with a pastry blender until crumbly; add buttermilk, stirring until dry ingredients are moistened.
Turn dough out onto a lightly floured surface; knead 3 or 4 times. (Don't overwork the dough.)
Place shortening in a 10" cast-iron skillet. Place skillet in 450° oven for 5 minutes.
Meanwhile, pat or roll dough to 1/2" thickness; cut with a 2 1/2" round cutter, and place biscuits in hot skillet.
Brush biscuits with 2 tablespoons melted butter. Bake at 450° for 14 minutes or until tops are golden.
Note: If you don't have a cast-iron skillet, biscuits bake perfectly on an ungreased baking sheet at 450°, omitting the shortening.