Stir the yeast into the buttermilk and let it sit for 10-15 minutes. In a large bowl, whisk together flour, sugar and salt and let sit for 10 minutes.
Add buttermilk, yeast and ¼ cup butter (melted) to the bowl and use a rubber spatula to mix it together until combined. At this point the dough will be wet and sticky. Cover and let rise somewhere warm for 2 hours until it has doubled in volume.
Uncover and sprinkle dough with a generous amount of flour then, using your hand or a dough scraper, turn it out onto a floured surface. Using a dough or bench scraper, divide the dough into 2 halves, then divide each half into thirds for 6 pieces. Divide each section in two again so you’re left with 12 pieces. Next, using as much flour as you need, shape each piece into a ball, smoothing the top and pinching the sides under the roll to hide the seam. Place rolls in a lightly greased 9x13-inch baking dish, spacing them evenly apart. (If you're not planning to bake them right away, this is when you should cover them and place them in the refrigerator.) Cover and let rise for 60-90 minutes until the rolls have doubled in size.
Bake at 400ºF (200ºC) for 25-30 minutes until golden.
While the rolls are baking, melt the remaining tablespoon of butter. Remove dish from oven and brush the tops with melted butter. Sprinkle with flaky salt and serve warm.