Preheat oven to 350 F. Line the bottom of two  6″ straight edge cake pans with parchment paper. Spray the sides with cooking spray.

In a large bowl, whisk to combine the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.  

In a medium sized bowl, whisk the egg, oil, sugars, buttermilk and vanilla extract until combined. 

Use a rubber spatula to gradually stir the wet ingredients into the dry ingredients. Stir until just combined, being careful not to over mix. Pour in the hot water and stir until combined.

Pour the batter evenly into the prepared pans and bake 28-32 minutes or until a toothpick comes out clean. Place pan on a wire rack and allow to cool in the pan for 30 minutes then remove from pan and cool on a wire rack. 

In the bowl of an electric mixer fitted with a paddle attachment or in a large bowl using a hand held mixer, cream the butter for one minute. Add the sifted powdered sugar and beat on medium for 3-4 minutes, until creamy. Add a half a tablespoon of milk or heavy cream. Slowly beat in the dulce de leche a little bit at a time. Beat in the vanilla followed by the salt. 

If buttercream is too thin, add more powdered sugar, if it is too thick, add more milk or heavy cream until it is the consistency you desire. 

Use a long serrated knife to trim the tops off the cooled cakes. Place first cake cut side up and frost the cake. Drizzle with a tablespoon or two of dulce de leche if desired. Place second cake cut side down on top of the first layer. Check to see that the layers are even. Top second layer with frosting and enjoy!