Preheat your oven to 350 degrees Fahrenheit.
Next, prepare your bundt pan by greasing it well with 2-3 tsp butter or margarine and "flouring" it with cocoa powder. Tap our as much of the excess cocoa powder as you can - you only want a very thin layer to coat the inside of the cake pan so it disappears as the cake bakes.
After the bundt pan is greased and floured, measure the flour, white sugar, brown sugar, cocoa powder, baking soda, baking powder and sea salt into a large mixing bowl. Give the mixture a stir with a wire whisk to combine all the ingredients.
To a separate bowl add the buttermilk, vegetable oil, eggs, and vanilla and whisk them together with a fork.
Dump the wet ingredients into the dry ingredients and stir them together with a rubber spatula to combine.
Set your kettle to boil and once it's boiled, measure out one cup of boiling water.
Add the boiling water to the cake batter in a slow stream, whisking as you add it. Continue adding the boiling water and whisking until the boiling water is all combined and the batter is smooth.
Pour the batter into the prepared bundt pan and bake at 350 degrees Fahrenheit for 36-40 minutes, or until a toothpick inserted into the cake comes out clean.
Let the cake cool almost completely on a wire rack in the bundt pan. While it cools it should pull away slightly from the pan, making it easy to remove when it's cool.
Carefully flip the bundt pan upside down on top of a wire rack to remove the cake.
Add some parchment paper under the wire rack once you've inverted the cake (this will catch the extra glaze you're about to make!).
In a medium bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.
Slowly pour the glaze over the cake, using a pastry brush to make sure every last inch of the cake is covered in the glaze.
Let the glaze dry completely before slicing into the cake to achieve that perfect, crispy, sugary crust!
Serve the cake as soon as the glaze is dry, and enjoy!