In a medium bowl, dissolve the yeast in the soy milk. Let sit 5 minutes.
In the bowl of a mixer, with the dough hook, combine the dry ingredients: the flour, salt, and sugar. Mix.
Add the mashed sweet potato to the yeast-milk mixture, and whisk to combine.
Add the wet ingredients to the flour mixture and mix on low until the dough begins to incorporate, about 3 minutes.
Add the softened butter little by little and increase speed to medium. Mix for 15 min. until the dough has come off the sides of the bowl and is smooth and stretchy, but not sticky. (If the dough is too sticky add a little more flour.)
Place the dough in a large oiled bowl, cover with a towel and let rise for 1 hour, or until doubled in size. Punch down the dough and fold the side over unto each other and flip. Cover the bowl with plastic wrap and refrigerate overnight.
The next day take the dough from the fridge, remove the plastic wrap and cover with a towel. Let rise in a warm place (70-75F) until the dough comes to room temperature, about an hour.
Divide the dough into 10 pieces weighing about 2.5 oz (70 g.), and set aside, roll them tightly into rounds and place on a sheet tray lined with parchment. Press down on the rounds to form a dome shape.
Make topping: Cream butter and sugar with a whisk or hand mixer. Add vanilla, flour, and cinnamon and mix well. Knead lightly to fully incorporate. It should have the consistency of a soft play-dough. If it’s too sticky or wet add flour in small amounts until you’ve reached the right consistency.
Divide topping into 10 balls the size of a large gum-ball. Take two sheet of plastic wrap, and one at a time place a ball of the topping between the two plastic sheets and press down with hand until it is large enough to cover the top of concha.
Peel one side of the plastic wrap off, then take the piece of plastic wrap with the topping on it and place it down on the concha. Slowly peel of the plastic wrap. Repeat this process until all 10 conchas are done.
Using a concha cutter, dusted with flour, press down on the topping and the concha to make the design. (Don’t be afraid to press down hard and flatten the concha a little.)
Preheat oven to 350°F.
Let conchas rise for 40 min. in a warm place (70- 75F) or until doubled in size.
Bake on the middle rack of the oven for 20 -25 min. or until the sides and bottoms of your conchas are golden brown.
Let bread cool completely before eating.