In a large bowl, stir together the yeast and warm water.
Mix in the evaporated milk, 1/3 cup sugar, salt, 1/3 cup butter, egg and 2 cups of the flour.
Stop mixing and scrape down the sides of the bowl.
Gradually add in the remaining flour and 1/2 teaspoon cinnamon.
Turn the dough out onto a floured counter.
Knead for 6 to 8 minutes, until smooth.
Place the dough in a greased bowl.
Turn the dough to coat the bottom.
Cover with plastic wrap or a kitchen towel.
Let rise in a warm place until it has doubled in size. About 1 hour.
Meanwhile, make the topping.
In a bowl, beat 2/3 cup powdered sugar and 1/2 cup butter until light and fluffy.
Scrape the sides of the bowl, making sure to mix everything.
Stir in the flour, cinnamon, and vanilla.
Mix until the mixture is the consistency of a thick paste. It will look like wet sand.
Divide into two parts, and place one part in a separate bowl.
If making chocolate conchas, add cocoa powder to one of the bowls, and mix until fully incorporated.
When the dough is done rising, cut into 12 pieces.
Make sure the dough pieces are the same size, or they will not bake evenly.
Shape the dough into balls.
Place on a greased cookie sheet.
Space the dough pieces out about 3 inches apart.
Gently press them down with the back of your hand.
Make 6 balls of the chocolate topping mixture and 6 balls of the white topping mixture.
Place each ball between wax paper.
Using a rolling pin, roll out the balls enough to cover the dough balls.
Place the rolled out topping on top of the dough balls.
Pat down lightly.
Use a knife to cut the grooves in the topping like a shell or criss cross.
Cover and let rise until doubled. About 45 minutes.
Preheat the oven to 375 degrees F (190 degrees C).
Bake for 20 minutes.