Prepare two 9 by 5 loaf pans (affiliate link) with parchment paper.
The next morning, make the Babka filling. Mix together the softened butter, brown sugar, cinnamon and flour to make a thick paste. Set aside. See recipe notes for strawberry version.
Pull the dough out of the refrigerator. Lightly flour a pastry mat or silpat liner (affiliate link). Turn the dough out onto the mat and cut in half.
Lightly flour one piece of dough and roll into a 14 by 18 inch rectangle.
Divide the Babka paste in two. Use your fingers to spread half the cinnamon sugar mixture over the rectangle of dough. Once the dough is covered, roll the dough up cinnamon-roll style.
Using a sharp knife, cut the roll in half; straight down the middle of the roll the long way. This will leave two long ropes of dough. Beginning on one end, squish the ends of the dough together and then twist the dough around each other forming the Babka loaf. Push the ends of the dough together and place the Babka in the prepared loaf pan. Repeat this process with the second loaf.
Cover the loaves and let rise 6-8 hours, depending on the warmth of your kitchen. This bread is made with 100% sourdough and will take much longer than a traditional loaf of bread to rise. Let it rise until it has mostly filled the loaf pan and gets light and puffy.
Preheat the oven to 350 degrees Fahrenheit. Whisk together the egg and a teaspoon of water. Lightly brush the egg wash on top of the bread until covered.
Bake Babka for 50-55 minutes until baked all the way through.
While the Babka is baking, prepare the simple syrup for the top of the Babka. Mix together the granulated sugar and water in a microwave safe liquid measuring cup. Microwave on high a minute at a time until boiling and all the sugar is dissolved. Mix together with a spoon and let cool until the Babka is out of the oven. The simple syrup can also be made on the stovetop. Boil the sugar and water together for about 1 minute until the sugar is completely dissolved. Cool a bit while the Babka bakes.
Pull Babka out of the oven and let rest in the pan for about 5-10 minutes. Remove the loaves from the pans using the parchment paper and place on a cooling rack. Pour half of the simple syrup on one loaf and half on the other loaf, using a pastry brush as needed. Let it soak into the warm bread.
Let the bread cool before slicing and enjoy!