In a bowl fitted for a stand-mixer, add flours, yeast, salt, sugar, water, and eggs. Attach dough hook and mix ingredients on low for 3-4 minutes until all the ingredients are combined, scrapping down the sides, as needed, to incorporate all the ingredients. Once incorporated, continue mixing for another 3-4 minutes. The dough will seem dry, but don't worry.
Cut butter into 12 pieces. Add one piece of butter to dough and mix on low until completely incorporated into dough. Repeat with remaining pieces of butter, 1 piece at a time. Once all butter is added, continue to mix on low for another 10 minutes. It is important for all the butter to be thoroughly mixed into the dough.
Once the butter is completely mixed into the dough, increase the speed to medium and beat for 15 minutes, until the dough is soft and slightly sticky. Don't worry - it will take time for the dough to come together so it will take the full 15 minutes. The dough will start of really loose and shaggy but will come together in the last couple of minutes. Once it's finished, beat on high for 1 minute. You should hear the dough make a slap-slap-slap sound as it hits the sides of the bowl. It will be ready once you can grab it all up in a ball. If it's still too loose, continue mixing for 2-3 more minutes on medium.
Put the dough in a large bowl or plastic container and cover it with plastic wrap, pressing the wrap directly onto the surface of the dough to prevent it from drying out. Let the dough proof in the refrigerator for at least 6 hours or up to overnight.