Note: A heavy stand mixer is needed for this recipe.
Combine 2 cups flour, undissolved yeast, water and eggs in a large mixer bowl. Beat 1 minute at high speed of electric stand mixer until smooth. Cover and let rest 45 minutes.
Add remaining flour, sugar and salt. Beat 8 minutes at medium speed using the paddle. Scrape bowl. Cut butter into 1/2 inch slices. Beat in butter using medium speed of mixer, a few slices at a time, until fully incorporated (This will take about 5 minutes.). Cover bowl and let dough rise in refrigerator for a minimum 4 hours or overnight.
Cut dough into 2 equal pieces. Shape and place each half into a 9 x 5-inch loaf pan coated with nonstick spray. Cover with a kitchen towel and let rise in a warm place until doubled in size, about 3 hours.
Whisk egg yolk and 1 tablespoon water until thoroughly combined. Brush over tops of loaves.
Bake in preheated 375°F oven for 35 to 40 minutes until golden brown (internal temperature in center of loaf should reach 190°F). Let cool 5 minutes in pan. Remove from pans by running a knife around the edges and invert onto wire rack to cool completely.