Combine ⅓ cup of the flour and the water in a small saucepan and place over medium heat, whisking constantly, until a thick paste forms (it should be a little looser than a roux), about 3 to 5 minutes. Reduce the heat to low, add the heavy cream and honey and whisk until honey dissolves. Remove from the heat.
Transfer the mixture to the bowl of a stand mixer fitted with a dough hook and add the milk powder, yeast, kosher salt, eggs, and 5 remaining cups flour. Knead on medium speed until dough is smooth, about 5 minutes. Add butter, a piece at a time, fully incorporating into dough before adding the next piece, until dough is smooth, shiny, and elastic, about 4 minutes (the dough may look loose and separated at first, but it will come together, just keep kneading).
Coat a large bowl with nonstick cooking spray and transfer dough to bowl, turning to coat. Cover with plastic wrap and let rise in a warm, draft-free place until doubled in size, about 1 hour.
Coat two 9- by 5-inch loaf pans with nonstick cooking spray. Turn the dough out onto a floured surface and divide into 12 pieces. Nestle the pieces side-by-side to create 2 rows down length of each pan. Cover with a clean dish towel.
Let the dough rise in a warm, draft-free place until doubled in size (dough should be just puffing over top of pan), about 1 hour.
Preheat oven to 375 degrees F. In a small bowl, whisk together the egg and water for the egg wash. Brush top of the dough with egg wash. Sprinkle with flaky sea salt, if desired. Bake, rotating pans halfway through, until bread is deep golden brown and an instant-read thermometer inserted into the center registers 190 degrees F, about 30 minutes. Let the bread cool in the pans on a wire rack for 10 minutes before turning them out, then let cool completely before slicing.
Notes
To make rolls, divide the dough into 12 equal pieces, shape into rolls, and place in greased 9x13-inch pan. Cover with clean dish cloth and let raise until doubled in size. Brush with egg wash and sprinkle with flaky sea salt, if desired. Bake for 15 to 20 minutes, or until golden brown brown and instant read thermometer registers 190 degrees F.
To make split-top rolls, lightly coat two 9x13-inch baking dishes with nonstick cooking spray. Divide dough into 12 pieces and shape each into a 4-inch long log. Place 6 logs in a row down length of each dish. Cover with clean dish cloth and let raise until doubled in size. Brush with egg wash and sprinkle with flaky sea salt, if desired. Bake for 20 to 25 minutes, or until golden brown brown and instant read thermometer registers 190 degrees F.