Preheat the oven to 350 F. Thoroughly butter a 9X13 baking pan or 2 8X8 baking pans. Be sure to get the corners! Set aside.

Whisk together the flour, both sugars, baking soda and salt in a large bowl. Set aside.

In a medium saucepan over medium high heat bring to a boil the cocoa powders, the butter and the water. Stir constantly. As soon as the butter melts and the mixture comes to a boil, pour the mixture into the bowl with the flour.

Using a handheld beater, beat the cocoa mixture into the flour mixture on medium speed just until mixed. Scrape the sides and bottom of the bowl and then add the eggs, buttermilk and vanilla. Beat on medium speed until smooth. Be sure to scrape down the sides and bottom of the bowl one more time before finishing.

Pour the batter into the prepared pan/s. Bake for 20-25 minutes (keep a close eye on the cakes if you are using smaller pans, but I found they took about the same amount of time as the bigger pan would). When they are done, the cake will start pulling away from the edges of the pan and a cake tester inserted into the center of the cake will come out with only a few crumbs attached (or clean).

Let the cake cool completely in the pan.

For the glaze, whisk together the glaze ingredients in a small bowl. Taste for additional salt, sugar or even buttermilk. Because this is a glaze and not a frosting, if the recipe as written will be too sweet for you, just add less sugar.

When the cake is cool, brush the glaze all over the cake, being certain to get the sides and corners as well. Let the glaze sit for at least 15 minutes before serving.