Bring ingredients to room temperature
Preheat oven to 350°F
Prepare 11x7 inch pan (grease and flour or parchment)
In a bowl, whisk the cocoa powder into the sour cream until smooth. Let the mixture rest for 5–10 minutes to allow the cocoa to hydrate, which will help it blend more easily into the batter.
In a separate bowl, whisk together the dry ingredients: flour, sugar, baking soda, and salt.
Whisk the eggs and vanilla together.
Add the egg mixture to the cocoa-sour cream mixture; stir until fully incorporated.
Gently fold the flour mixture into the wet mixture in two additions until nearly combined (it's ok if some streaks of dry ingredients remain).
Pour into the prepared pan.
Bake for 25-35 minutes; the center should spring back to the touch and some crumbs remain on a toothpick (but not wet batter).
Remove from the oven and allow to cool on a wire rack until the pan is cool enough to handle.
Invert the cake on the wire rack and allow to cool completely before icing (if you like, once cooled you can put it back in the pan and frost for greater portability).
Frost with an icing of your choice; I chose a cream cheese icing but of course any kind of icing would be fantastic!