Bloom the yeast: Add yeast to lukewarm water, stirring to dissolve, and leave at room temperature for 10 minutes so the yeast can "bloom," or activate. Meanwhile, heat milk in a small saucepan just until warmed through, but not hot. Remove from heat and stir in the molasses and butter. Cool slightly.
Proof the dough: Add the flour to the bowl of a stand mixer. Using a dough hook on low speed, add milk mixture, salt, buttermilk, and the bloomed yeast. Mix until incorporated. Scrape down the sides of the bowl and continue mixing until the dough comes together and takes on a glossy consistency. Place the dough in a bowl covered with plastic wrap (or a container with a tight-fitting lid) and let it rest in a warm place (68-74 degrees F) for 1—1½ hours, until the dough has doubled in size.
Roll the dough: Once the dough has doubled in size, punch it down and place onto a lightly floured surface. With a floured rolling pin, roll out the dough until it's about 1/3-inch thick. Using a 3-inch biscuit cutter (or drinking glass), cut circles out of the dough (use remaining dough to make more rolls). Using the flour-dusted rolling pin, indent the middle of each roll to make ovals. Fold the ovals in half, pinch to close, and place on a parchment-lined baking sheet. Cover with a dry kitchen towel and let the rolls rest in a warm place until they've doubled in size, 45 minutes.
Bake the rolls: Heat oven to 425 degrees F. Place rolls in the oven and bake until golden brown, 15—18 minutes. Remove and brush with melted butter and a sprinkle of flaky sea salt; serve piping hot. Note: If you've made extra rolls and want to freeze some, bake as directed, but don't finish with melted butter or salt. Cool on the sheet pan, then place in a plastic freezer bag and put the bag into the freezer for up to two weeks. To thaw rolls, remove from the freezer the night before you want to serve them. Take the rolls out of the plastic bag and wrap loosely in foil. Let thaw at room temperature overnight. Rolls can be served as soon as they've thawed, but are even better if re-warmed in their foil packing for about 10 minutes in a 300 F oven. They can also be microwaved (out of the packaging) directly from the freezer for 15—30 seconds, depending on your microwave. Finish with melted butter, salt and serve.