Preheat oven to warm, 170 degrees F.
Place butter, buttermilk, and water into a heat-proof bowl. Microwave until butter is mostly melted and between 115-120 degrees F, about 30-45 seconds.
In a stand mixer with dough hook attached, place flour, sugar, yeast, and salt, mix to combine. With the mixer running, slowly add warm milk mixture. Increase speed to medium and knead for 4-5 minutes, using a rubber spatula to scrape edges of the bowl. The dough should be slightly sticky but manageable.
Drop dough onto a lightly floured surface. Shape into an evenly level square, about 9 by 9-inches. Cut into 16 equal portions (don't have to be exact). Shape dough into 15-16 balls, while using excess dough from the 16th portion to add to smaller rounds as needed, and place shaped dough into prepared baking dish.
Brush lightly with melted butter, just to keep them moist while rising.
Transfer to oven, close oven and turn oven off, allow rolls to rise 20 minutes (don't open oven door). Remove the pan from the oven and preheat oven to 375 degrees.
Brush again with melted butter and sprinkle with kosher or sea salt.
Bake for 14-16 minutes until tops are golden brown.