Preheat oven to 350 F. Line a muffin pan with cupcake liners and line another muffin pan with 2 liners. This recipe yields 14 cupcakes.
In a bowl, whisk together cocoa powder, flour, baking powder, baking soda and salt. Set aside. In another mixing bowl, whisk eggs, sugar,brown sugar, vanilla and oil. Add half of this liquid mixture to the dry ingredients. Add half of the buttermilk. Stir until combined. Repeat the same manner of addition with the remaining wet ingredients and buttermilk. Stir the mixture just until combined. Do not overmix. Batter will be thin.
Pour the batter into the prepared muffin pans until they are HALF full. Bake in the preheated oven for 18-21 minutes, or until a toothpick inserted at the center of cakes comes out clean with some moist crumbs. Cool the cupcakes on a wire rack.
Make the Yema Frosting
In a medium saucepan, stir together cornstarch, sour cream and condensed milk until smooth. Cook the mixture over medium heat, stirring constantly and cleaning the sides of the pan often.
The mixture will start to thicken and occasional big bubbles will start to surface. Turn off heat. The mixture will be thick as in when you lift the spoon, the mixture will fall back down in chunks or dollops into the pan.
Lightly beat egg yolks in a bowl. Add about 1 cup of the hot mixture into the yolks and stir rapidly. Pour this mixture back to the hot pan. Cook everything again over medium heat, until the mixture forms occasional big bubbles and the consistency is spreadable. Turn off heat.
Let the mixture cool to room temperature and use a metal spatula to spread the frosting on top of the cupcakes.