Preheat the oven to 350 F.
Grease and flour a 12 cup bundt cake pan, ensuring all the sides, corners and nooks are covered.
Use a large bowl and a hand mixer, or a stand mixer (with the paddle attachment) to make the batter: set the sugar, eggs, salt and vanilla in the bowl and begin beating on low, working up to medium speed until the mix is light, airy and fluffy - about 5 minutes. It should fall from the beaters back into the bowl in ribbons.
With the mixer on, slowly pour in the oil in a steady stream. Once all the oil is in, beat the batter for 30 seconds to fully combine.
Sift in the dutch cocoa and beat to combine.
Sift in the cake flour, add the baking soda and baking powder. Turn the mixer on low then slowly pour in the buttermilk and beat to combine. Use a rubber spatula to scrape the sides of the bowl as needed (and check the bottom of the bowl too) to make sure everything is evenly mixed.
Slowly pour in the boiling hot water and beat to just combine, repeat scraping the sides and bottom of the bowl and beat again to just combine.
Pour the batter into the prepared bundt pan. Bake until a cake tester comes out clean or the top springs back when pressed, about 45 minutes.
Once cool enough to handle, invert the bundt cake onto a plate (ideally you invert it twice so that it’s upright - the way it baked - again. This will give the glaze more areas to gather rather than run off the cake.)
While the cake is still warm, make the glaze by whisking all the ingredients together. If it’s too thick to spread smoothly, add a couple more tablespoons of heavy cream.
Spread the glaze over the cake in a thick layer at the top; the heat of the cake will make the glaze melt and run down the sides and middle. Serve once slightly cooled or later that day.
Store leftovers in an airtight container. The cake will keep for about 2-3 days.