In a big bowl, sift the flour, corn starch, baking powder, baking soda and salt together. Mix and set aside.
In a different big bowl, using a hand mixer, cream together the butter, oil and sugar for 2 minutes until you get a light and fluffy mixture.
Add in the eggs one at a time, beating after each addition, then beat in the vanilla extract and the vanilla bean seeds and beat for another 3 minutes. The mixture must turn really fluffy.
Incorporate the dry ingredients in three separte additions, alternating with the buttermilk, starting and finishing with the dry ingredients (once you´re done with the last dry ingredients addition is important not to beat too much, just until the mixture is well incorporated).
Distribute the batter filling a cupcake pan (already prepared with cupcake liners) about 3/4 each.
Bake at 350°F for 22 min. aprox. or until a wooden stick comes out clean when inserted.
Put the dulce de leche in a pastry bag with a small piping tip. Once the cupcakes are cool, take out a small piece of each using a knife and fill them with dulce de leche, then put the small pieces of cake back in their places.
For the frosting:
In a big bowl, with a hand mixer beat the butter and cream cheese together until soft.
Add in the confectioners sugar and salt (both previously sifted) then the vanilla extract and the vanilla bean seeds and continue beating until the mixture lightens in color.
Beat in the milk and keep on beating until you get a fluffy frosting that´s almost white.
Decorate the cupcakes with the frosting however you like.