Preheat oven to 350 degrees F. Line a 9 x 13 pan with foil, leaving an overhang on the ends for easy removal.
In the bowl of a stand mixer, combine the butter and sugar until smooth.
Add all four eggs, and mix until fluffy and well-combined.
Add the salt, cocoa, and vanilla, mix until combined.
Add the flour and mix on low speed until mixture is combined. It will be thick.
Push batter into prepared pan and bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
Remove from oven and set aside to cool before frosting.