Preheat your oven to 350 degrees Fahrenheit.
Prepare your bundt pan by buttering the inside and coating it with cocoa powder. Tap out any extra cocoa powder.
Set a saucepan over medium heat and add the 1/2 c. of salted butter. Once melted, whisk in the cup of cocoa powder. Once combined, whisk in the water and turn off the heat. Whisk in the buttermilk, sugar, vanilla, and eggs.
In a large bowl, sift together the flour, baking soda, cinnamon, and cayenne pepper. Slowly pour the liquid chocolate mixture into the bowl while continuing to whisk.
Once the batter is well-combined, pour it into the prepared bundt pan and bake 40-45 minutes or until a toothpick comes out clean. Cool on a rack for 15 minutes, then tip the cake out onto a plate. Let cool for another 10 minutes.
Meanwhile, make the glaze by melted the unsalted butter in a saucepan over medium heat, then adding in the cream, powdered sugar, and chocolate. Stir until smooth, then turn off the heat and let cool for 5 minutes.
Pour the glaze over the top of the bundt cake, then garnish with chocolate curls, sliced almonds, or finely chopped Macadamia nuts.