Preheat oven to 350 degrees. Prepare 1 9x5 inch loaf pan by spraying thoroughly with non-stick spray. Set aside. In a small bowl, whisk cocoa powder, flour, salt, and baking soda together until combined. Set aside until the final step of making the batter.
In a bowl of a stand mixer, add sugar and oil and mix until combined. Add egg and egg yolk, together and mix on medium-low until combined. Add vanilaa extract and mix together. Make sure to scrape the bottom of the bowl to ensure even mixing.
Mix coffee together with sour cream in a separate mixing bowl or measuring cup. Add ⅓ of the flour mixture to the batter and mix on low until combined. Add half of the coffee-sour cream mixture and mix on low until smooth. Repeat with another ⅓ of flour mixture and then the last of the coffee-sour cream mixture. Finally, add the last of the flour, mix, and stop when combined. Pour batter into prepared 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 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, about 40 more minutes.