For the dough: In a small (2-quart) saucepan set over medium-low, heat the milk until it is warm to touch (110 degrees or a little more than body temperature). Measure the yeast and 1 teaspoon of the sugar into a small bowl or measuring cup. Pour the warm milk over the yeast, stir well and let it sit for about 10 minutes until the yeast is active and foamy (if the milk is too hot, the yeast will not become active).
In another bowl mix the flours, 1 tablespoon of the sugar and the salt.
When the yeast is foamy pour it into the bowl of a stand mixer fitted with the dough hook. Add eggs and mix at low speed until thoroughly combined.
With the mixer running, very slowly start adding the flour mixture. Continue mixing until it is a smooth, elastic dough, scraping the sides of the bowl as needed. Add the cubes of butter one at a time, making sure that each piece is incorporated before adding the next.
When the dough is well mixed, place it in a glassbowl. Cover with plastic wrap directly on the surface of the dough and allow it to rise in the refrigerator overnight. If you don’t have that much time, cover the bowl with a kitchen towel, place in the warmest part of your kitchen and allow it to rise for about 2 hours, until the dough has doubled in size. Then remove the towel and cover the dough with plastic wrap directly over the dough’s surface and refrigerate for 1 hour to chill it thoroughly.
For the streusel topping: While the dough is rising, scoop the butter and sugar into the bowl of the stand mixer fitted with a paddle and mix at medium speed until thoroughly combined. Reduce the speed to lowand slowly add the flour and cinnamon until the mixture is smooth. Transfer to a small bowl, cover and refrigerate until ready to use.
For finishing the conchas: Line a baking sheet with parchment paper. When the dough has doubled in size, transfer it to a lightly floured surface. With a knife or bench scraper, divide the dough into 9 even pieces. One by one, cup your lightly floured palm over a ball of dough and roll in a circular motion while pressing gently against the ball. This should create a beautiful sphere. Evenly space the dough balls on the baking sheet, cover with a kitchen towel, move to a warm part of your kitchen and allow the balls to rise until doubled in size (1 to 2 hours depending on the temperature of your kitchen).
Turn on the oven to 350 degrees. Divide the streusel topping into 9 pieces and flatten each one with your hands, a rolling pin or a plastic lined tortilla press to form uniform, flat disks that are large enough to cover the risen dough balls. When ready to bake, beat the egg with 1 tablespoon of water, brush the conchas with the egg wash and lay a tortilla-shaped piece of streusel topping over each concha, pressing it lightly onto the surface. Using a sharp pairing knife, slice lines just through the streusel layer to create the traditional shell pattern. Bake for 25 to 30 minutes, rotating the baking sheet halfway through.