George and I make them often, but they never turn out right. We affectionately refer to ours as "chocolate rocks." I wouldn't advise following our recipe, but I stand by the instructional video.
L&G's Chocolate Rocks
8 oz chocolate chips
6 T unsalted butter
6 T unsalted butter
4 T cocoa powder
2 egg yolks
1/4 c flour
2 t vanilla
Melt butter, chocolate, and cocoa over medium heat.
While chocolate cools, beat yolks, flour, and vanilla.
Beat together and bake for 9 minutes at 400 in muffin tin.
"You're Never Fully Dressed Without a Smile" by Charles Strouse.