Preheat oven to 350ºF.
Prepare a Bundt® cake pan or three 8" round cake pans with baking spray. If using cake pans, add parchment paper rounds to the bottom of each after spraying each with the baking spray.
Place the cocoa powder and cayenne pepper in a medium bowl and add the hot water. Whisk until smooth and set aside to cool.
On high speed in a stand mixer or using a hand blender, blend the vegetable oil and sugar together. Add the buttermilk. Continue mixing until fluffy. Beat in the eggs and egg yolks until all are fully incorporated.
In a separate bowl, whisk together the flour, baking soda, baking powder and salt. Set aside.
Add the cooled chocolate mixture to the batter. Slowly add the dry ingredients to the batter, mixing well on low speed, until smooth.
Pour batter into the fluted pan or evenly divide the batter between the three prepared cake pans. Bake at 350ºF for 25 minutes or until tester when inserted in the center comes out clean. While the cake is baking, make the frosting.
When cakes are finished, remove from the oven and let cool on racks for no longer than 10 minutes. Turn cakes out of the pans onto racks, peel off the parchment paper, if using. Let cool completely.