How to Make It
In a large bowl, mix the all-purpose flour and bread flour. In a small sauce- pan, heat 3/4 cup of the milk until lukewarm. Pour the warm milk into a medium bowl and stir in the yeast. Let stand until the yeast dissolves and the milk becomes creamy and starts to bubble, about 5 minutes. Stir in 1 1/2 cups of the mixed flours. Cover with plastic wrap and let the sponge stand until slightly risen, about 20 minutes.
Scrape the sponge into the large bowl of a standing electric mixer fitted with a dough hook. Add the sugar, salt, eggs and the remaining 2 cups of milk and beat at medium speed until blended. Turn the machine off and carefully add the remaining 6 1/2 cups of flour. Beat at low speed until the flour is just incorporated, then knead at medium speed until the dough is evenly combined, about 5 minutes, scraping down the sides of the bowl occasionally. Add the butter and continue to knead until the dough is very soft and silky, about 10 minutes longer.
Lightly butter a large bowl. Transfer the dough to the bowl and cover with a sheet of oiled plastic wrap. Let the dough stand in a warm, draft-free spot until doubled in bulk, about 1 hour. Punch down the dough, then cover and let the dough rise until doubled in bulk again, about 1 hour longer.
Punch down the dough and turn it out onto a heavily floured work surface. Divide the dough into 3 equal pieces and let rest on a lightly floured work surface for 20 minutes.
Butter three 8-by-4-inch loaf pans. Using lightly floured hands, form each piece of dough into a loaf shape about the same size as the pans. Brush off any excess flour from the loaves and gently ease them into the pans. Cover the pans loosely with oiled plastic wrap and let stand in a warm, draft-free spot until the dough rises 1 1/2 inches above the rims, about 45 minutes.
Preheat the oven to 350°. In a small bowl, mix the egg yolk with the cream. Brush the loaves with the egg wash and bake in the center of the oven for 1 hour, or until the tops are golden brown. Carefully turn the loaves out of the pans onto a wire rack. They should sound hollow when tapped on the bottom; if they don't, return the loaves to the pans and bake for a few minutes longer. Let the bread cool completely on wire racks before slicing.