Bring cream to a boil in a 1-quart saucepan, then pour hot cream over chocolate in a bowl, gently whisking until smooth.
Keep warm or at room temperature. Next, make griddle cakes.
Instructions for Griddle Cakes:
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 cast iron griddle and cook until bubbles appear on surface, 1 to 2 minutes.
Flip cakes with a large spatula and 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 teaspoon butter to griddle between batches.
Serve cakes in stacks, topped with chocolate sauce.