Preheat oven to 375 degrees.
Place a small metal bowl over a saucepan with simmering water. Melt the chocolate and butter in the bowl. Stir in vanilla.
In a large mixing owl, combine sugar, flour and salt. Sift these into the chocolate and mix well with electric hand mixer. Add eggs one at a time, fully incorporating each egg before adding the next. Beat at high until batter is creamy and lightens in color, approximately 4 minutes. Chill mixture.
Coat the top and each cup of the muffin tin with butter. Dust with the cocoa powder and shake out excess. Spoon mixture into pan using a 40ounce scoop or ladle. Bake for 10 to 11 minutes. Outsides should be cake-like and centers should be gooey.