To make the sponge, in the bowl of a stand mixer, whisk together 1 cup flour, the milk, 1 tablespoon sugar, and the yeast until combined. Loosely cover, and let stand at room temperature, until foamy, about 45 minutes.

Once the sponge is ready, add 6 eggs, the salt, remaining 5 cups flour, and remaining 5 tablespoons sugar. With dough hook attachment, beat on low speed until well combined, about 2 minutes scraping down the bowl as needed. Continue mixing until the dough is shiny and elastic, about 10 to 12 minutes.

With the mixer on low speed, gradually add in the butter 1 tablespoon at a time, waiting until it is fully incorporated before adding the next tablespoon. Once all of the butter is incorporated, increase the speed to medium-low and continue kneading until the dough is smooth and elastic, about 8 minutes more. (See Notes about trying the window pane test!) Scrape the dough down into the bowl. Cover, and let rise in a warm place until doubled in size, about 1 hour.

Lightly grease two 8¼x4½-inch loaf pans with softened butter.

Punch down the dough and turn out onto a lightly floured surface. Divide the dough in half. Working with one half at a time, cut the dough into 3 equal pieces, and roll each piece into a 9-inch long rope. Braid the ropes together, pinching ends together to seal. Tuck the ends under and place in a buttered loaf pan. Repeat with the remaining dough half. Loosely cover, and let rise in a warm place until doubled in size, about 1 hour.

While loaves are rising, preheat the oven to 375F.

In a small bowl, whisk together the water and remaining 1 egg until smooth. Brush the risen loaves with egg wash.

Bake for 35 minutes or until deep golden brown, covering with aluminum foil after 25 minutes if the top is browning too quickly. Let the bread cool in the pans for 5 minutes. Remove from pans, and continue cooling on a wire rack. Serve warm or at room temperature. Store cooled bread in an airtight container for up to 1 week, or slice and freeze in a freezer bag for up to 2 months.