In the bowl of a stand mixer with the dough hook attachment, add the eggs to the sponge/dough starter and beat on low for 1 minute, then raise the speed to medium for 2 minutes. Scrape down the sides of the bowl, and continue beating for 5 minutes. The dough will be soft and sticky. Add the butter one tablespoon at a time, making sure each tablespoon is incorporated before adding the next.
Scrape the dough into a lightly oiled bowl or container, and lightly coat the top of the dough with nonstick cooking spray. Cover the container with a lid or tightly wrapped plastic wrap. Let the dough rest in a warm place for about 2 hours, or until it has doubled in size.
After rising, refrigerate the dough for 1 hour to firm, then gently stir it, deflating it. Return to the fridge for another. This makes it less sticky.
Turn onto a well floured surface and form into a rectangle, either with your hands or a rolling pin (I used my hands). Fold it into thirds, like a business letter, and brush off any excess flour with your hands or a pastry brush in the process. Press it down/roll it again into another rectangle, then rotate 90 degrees so the closed end is facing your left. Fold it into thirds again and round the corners. Wrap it loosely in plastic wrap and refrigerate for 6 hours to 2 days.
Remove the dough and press down on it to deflate. Using a large knife, cut the dough into 8 equal pieces. Roll each piece into a golf-ball sized ball. Spray a 9"x5" loaf pan with nonstick cooking spray. Place the balls of dough side by side in twos. Let rise in the pan in a warm spot in your kitchen for 1 1/2 to two hours.
Whisk together the egg yolk and heavy cream for the glaze. Using a pastry brush, brush the top of the dough generously with the glaze.
Bake in an oven heated at 350 degrees Fahrenheit for 30-40 minutes, or until golden and fragrant. Let cool before turning out of the pan.