Preheat oven to 350 degrees. Prepare 10-inch Bundt pan by spraying thoroughly with non-stick spray. Set aside. In a small bowl, whisk cocoa powder and coffee together until combined. Set aside until the final step of making the batter.
In a bowl of a stand mixer, add sugar, salt, eggs, egg yolk, and baking soda together and mix on medium-low until combined. Add coffee mixture, oil, and vanilla extract and mix until combined. Make sure to scrape the bottom of the bowl to ensure even mixing.
Add half of the flour to the batter and mix on low until combined. Add buttermilk and mix on low until smooth. Finally, add the last of the flour, mix, and stop when combined. Pour batter into prepared Bundt pan and smooth into an even layer.
Bake in center of preheated oven for about 1 hour. Remove the cake when a knife comes out clean when inserted in the center. Let it cool for 20 minutes before removing from the pan. To remove from the pan. slide a knife around the edge to loosen up the cake. Place a plate on top of the Bundt pan and, with pot holders on, invert the pan so it's on top of the plate. Gently tap the pan on the plate to loosen up the cake. Remove pan and let cake sit to cool completely before icing, about 40 more minutes.