Preheat the oven to 325 degrees Fahrenheit. Spray or grease a 13x9 inch glass pan.
In a large mixing bowl, add the flour, cocoa powder, sugar, salt, baking powder and baking soda. Whisk well and set aside.
In another bowl, add the buttermilk, whole eggs, egg yolk, vanilla extract and vegetable oil. Whisk well and set aside.
Cut the room temperature butter up into pieces (if not already done). Turn the mixer on and add in the butter pieces slowly to the dry mixture while you mix on medium speed. (You can use a hand mixer or a stand mixer.) Once all the butter is added, mix until the flour mixture becomes crumbly and resembles sand.
Add in half the liquid mixture and mix on medium speed, for a full minute.
Add in the last half of the liquid mixture and mix only until incorporated. Scrape down the sides of the bowl and mix again just until all the ingredients are mixed well. Only mix for about 10 seconds or so at this point. Mixing for a minute or more is too long and can cause a dense or tough cake.
Pour batter into prepared pan and bake at 325 degrees for approximately 50-55 minutes.
Cake is done when a toothpick inserted into the middle of the cake, comes out with a few moist crumbs or clean. (You do not have to wait until the toothpick comes out clean as that can over-bake your cake. Just make sure there is no raw batter on the toothpick and a few moist crumbs are fine.)
Once the cake is baked, set on a wire rack to cool for about 15 minutes while you make the glaze. The cake may flatten out a bit on the top, which is fine.