Put oven rack in middle position and preheat oven to 350°F. Butter and flour your Bundt pan using cocoa powder, knocking out excess.

Melt butter in a saucepan over moderately low heat, then whisk in the cocoa. Add the water and whisk until smooth, then remove from heat. Whisk in separately sugar, eggs, buttermilk, and vanilla.

Sift together your flour, baking soda, cinnamon, cayenne pepper and salt into a bowl, then sift again into cocoa mixture and whisk until just combined.

Pour the batter into bundt pan and bake until a wooden pick or skewer comes out clean, about 45 to 50 minutes.

Cool cake in bundt pan on a rack for about 15-20 minutes, then loosen edges with a knife and invert onto a plate.

Melt butter in a saucepan over low heat, then stir in the heavy cream and confectioners sugar. Add the chocolate and stir, until smooth. Turn off the heat and cool glaze until slightly thickened, about 5 minutes.

Pour the glaze over top and sides of the cake and sprinkle the sliced almonds.