Make the sponge: Using the bowl of a stand mixer add the warm milk and yeast. Whisk until yeast dissolves. Add the sugar and flour. Mix on low using a paddle attachment until a thick batter forms. Cover with plastic wrap and let rest at room temperature for about 1 hour, or until bubbles form.
Make the dough: Add flour and salt to the sponge. Mix on low speed using the dough hook attachment. Add eggs and mix on low until just incorporated. Turn to medium speed and knead dough for 5 minutes.
Add the butter 2 Tbsp at a time. Knead until the dough is shiny and smooth and no butter is visible. Place the dough into a lightly oiled bowl. Cover with plastic wrap and let rise for 2 hours or until dough doubles in volume.
Punch down the dough and cover again with plastic wrap and let rise for another 45-60 minutes.
Transfer dough to a lightly floured surface. Roll out dough into a 18x13-inch rectangle.
Cut Butter Block into thin slices and place evenly over the center and right thirds of the dough rectangle. Fold the left edge of the dough to the right, covering half of the butter. Fold the right third of the rectangle over the center third, like a business letter. If butter falls out, just place it back into dough.
Place dough on a greased baking sheet and cover with plastic wrap. Place in fridge for 30 minutes.
Remove from fridge and place lengthwise on a lightly floured surface. The open ends should be to your right and left.
Roll out the dough into another 18x13-inch rectangle. Again, fold the left third of the dough to the right, covering the center third of dough. Fold the right third of the rectangle over the center third. No butter is added this time.
Place back on baking sheet, cover, and chill for another 30 minutes.
Repeat the folding process one more time for a total of three turns.
Wrap in plastic wrap and place in fridge overnight. You can also freeze it at this point for up to one month. Spray plastic wrap with baking spray to prevent dough sticking.
Make the filling: Place sugar in a bowl and add the lemon zest. Rub the zest into the sugar with your hands, releasing the lemon oil into the sugar.
Assemble the buns: One last time roll out the dough on a lightly floured surface into a 18x13-inch rectangle. Brush the melted butter using a pastry brush over the surface of the dough. Sprinkle the lemon sugar over the dough covering all but 1 inch at the top edge of the rectangle. Sprinkle the blueberries over the lemon sugar.
Starting at the long edge closest to you, begin to roll up the rectangle like a jellyroll. Roll tightly at first, then ease up and push the dough into a log. Using a serrated knife, cut the log crosswise every 1 1/2 inches, creating 12 spiraled buns. You can secure the rolls with a toothpick to hold their shape, if desired.
Place buns 2 inches apart, cut side up, on a prepared baking sheet (lined with parchment). Cover loosely with plastic wrap. Let rise at room temperature for about 45 minutes.
Preheat oven to 400°F.
Make egg wash: Whisk together the egg and egg yolk. Using a pastry brush, lightly coat each bun with egg wash.
Bake for 10 minutes at 400°F. Then turn down to 350°F. Bake another 10-15 minutes more or until the buns are golden brown. You will see a lot of butter bubbling away…don’t worry…it will soak back into the pastry. And that makes it oh so good.
Make the glaze: Add the ingredients together in a medium bowl and whisk until combined. If too thick add more milk. If too thin add a little more powdered sugar. Drizzle over buns.
Best if served warm. Enjoy!