Place the bread flour, buttermilk, brown sugar, salt, yeast and egg yolk into a bread machine. Using the Dough setting allow the machine to mix the ingredients, knead, and rise.
Grease a muffin tin or a baking tray and set aside.
Punch down the dough, and remove it from the machine. Divide the dough into 12 equal parts and form into round, smooth rolls. Place them into the muffin tin or onto the baking tray, making sure the rolls don't touch each other. Cover the rolls with a tea towel and let them rise in a warm place until they double in size (about 25 minutes).
While the rolls are rising, preheat oven to 170 C / Gas 3.
Bake the rolls in the preheated oven for 20 minutes, or until risen and golden brown. Brush the tops of the hot rolls with melted butter for a soft crust, cool the rolls slightly, and serve warm.