Sift together cocoa, flour, sugar, baking soda, and salt into a large bowl, then whisk in eggs, yolk, buttermilk, oil, and vanilla until combined well.
Heat a griddle or nonstick skillet over moderately low heat and lightly coat with butter.
Working in batches of 2 or 3, pour 1/4 cup batter per cake onto hot griddle; cook until bubbles appear on surface, 1 or 2 minutes. Flip cakes with a large spatula; cook until tops spring back when pressed gently, about 1 minute more.
Transfer to a plate and loosely cover with foil to keep warm. Add 1/2 tsp. butter to griddle between batches.
Optional: You can dress it up or down 3 ways. 1. With slices of oranges, strawberries, or raspberries. 2. Bring 1 cup heavy cream to a boil in a 1 quart saucepan. Pour over 7 oz bittersweet chocolate in a bowl. Whisk until smooth. Keep warm or at room temperature. 3. You can Top with your favorite ice cream flavors.