Make the Roux (Tangzhong): In a small saucepan, mix together the water, whole milk, and bread flour until combined. Cook over medium heat, stirring until a thick, white paste is formed, about 2 minutes. Cool for about 10 minutes.
To make the Dough: In a stand mixer fitted with the dough hook, mix together the remaining bread flour, salt, sugar, yeast, whole milk, buttermilk, egg, butter, and rosemary for 1 minute. Add the roux (Tangzhong) and mix until it comes together to make a uniform dough, about 5 to 8 minutes. Cover with a wet towel or plastic wrap and let rest for about 1 to 1 1/2 hours, or until it has doubled in size.
While the dough is rising, prepare the crockpot. Pad the bottom of a 6-quart crockpot insert with a piece of crumpled-up aluminum foil that is about 1/2-inch thick (this will protect the bottom of the rolls from over-browning), then line the entire insert of the crockpot with a larger sheet of foil and finally place a large piece of parchment paper over the foil (These steps prevent the dough from sticking to the sides as they rise and bake).
Once the dough is fully risen, turn it out onto a floured surface and divide into 8 to 12 portions. Roll into round buns. Place the buns into the prepared crock pot and "bake" on the "high" setting for about 1 1⁄2 hours, the timing will depend on your machine. Don’t uncover until the tops look set. They will be pale in color and may deflate a bit when uncovered, so check them quickly by tapping lightly on the top. Check the center buns. If they seem set and not like raw dough, they can be finished under the broiler.
Once they are done cooking in the crockpot, remove the rolls and place on a baking sheet. Brush with butter and broil in the oven for about 5 minutes to brown and finish the baking. Brush with more butter and flaky sea salt. Serve warm or at room temperature. Best served the day they are made.