In a bowl, stir together the milk, water, yeast and sugar. Let sit until foamy, about 15 minutes.
In a large bowl (or the bowl of your electric mixer – that’s what I use!), stir together the flours and salt. Take the butter and mix it into the flour using your fingers to make coarse crumbs.
Stir the yeast mixture and 1 beaten egg into the flours (or use your stand mixer on the lowest setting, just until a dough has formed. Knead the dough (either on a floured workspace with your hands, or in your stand mixer using the slowest settinfor 8 to 10 minutes. The dough will be sticky – don’t add more flour! You want it to be sticky.
Place the dough in a lightly oil bowl and cover it. Let rise for 1 to 2 hours until it has doubled in size.
Line a baking sheet with parchment paper. After the dough has risen, divide it into 8 or 10 equal parts (or even 12, for smaller bunand form them into buns. Place them on the baking sheet about 2 to 3 inches apart (they will rise!) and cover. Let them rise for another hour or so.
Place a pan of water on the bottom rack of your oven. Preheat the oven to 400 degrees F.
Brush the buns with the egg wash and sprinkle with the flaked sea salt. Bake for 15 to 20 minutes, turning the pan halfway through baking. Let cool completely before using.