Activate Yeast: In a large bowl, mix warm water and yeast. Let it sit for 5 minutes until foamy.
Prepare Dough: Add 2 cups flour, evaporated milk, sugar, melted butter, egg, salt, and cinnamon to the yeast mixture. Gradually add the remaining 2 cups of flour and mix into a dough.
Knead and Rise: Knead the dough on a floured surface for 6-8 minutes until smooth. Place in a greased bowl, cover, and let rise for 1 hour.
Make Topping: Cream together sugar and butter. Mix in flour to form a thick paste. Divide and flavor one portion with cinnamon and another with vanilla.
Shape Conchas: Divide the dough into 12 balls and place on a greased baking sheet. Flatten topping into circles and place over dough balls. Score with a seashell pattern.
Second Rise: Cover and let rise for 45 minutes.
Bake: Preheat oven to 375°F (190°C). Bake for 20 minutes or until golden. Cool slightly before serving.