In the bowl of an electric mixer, combine the water, buttermilk, butter, sugar salt, and yeast. Stir briefly with a spoon and let sit for 6 minutes, or until the yeast bubbles and foams.
When the yeast is well-foamed, add 5 cups of flour and mix on low speed with the paddle attachment until a rough, shaggy dough forms.
Switch to the dough hook and turn to low speed. Add additional flour 1/2 cup at a time until a ball begins to form around the dough hook. (I used about 3/4 cup additional flour.) When the dough begins to pull away from the sides of the bowl, knead on medium-low speed for 5 minutes. The dough should form a smooth ball and feel tacky to touch.
Spray a large bowl with baking spray and turn the dough into the bowl. Lightly spray the top of the dough with the baking spray and cover with plastic wrap. Let rise 30 minutes in a warm place, until dough is doubled.
Deflate the dough with your hand and turn it onto a lightly floured surface. Pat the dough into a rectangle 8×12 inches. Cut the dough into four strips lengthwise, and then cut the strips into six portion creating 24 squares. Roll the squares into balls using your cupped hands.
Place all 24 rolls in a half sheet pan, or place 12 rolls in each of two 13×9-inch baking pans. Space them about 1/2-inch apart. Cover with plastic wrap and let rise for 20 minutes, or until the rolls are well-puffed.
Preheat oven to 350°F. Bake the rolls for 20 minutes, or until the tops of the rolls turn golden brown. Brush the warm rolls with melted butter before serving. Serve warm.
To freeze unbaked buns, allow them to rise for 10 minutes before freezing them on their pans. Transfer frozen rolls to freezer-safe zip-top bags. Keep frozen for up to 4 weeks. When ready to use, place on pans and cover with plastic wrap; thaw overnight in the refrigerator. Bake as directed.