Preheat oven to 350 degrees. Melt the butter in a saucepan at low temp. In a bowl, mix the eggs, sugar and vanilla thoroughly. Sift the flour and cocoa into the sugar mixture and the add the melted butter - mix well. Pour the mixture into 12 - 1/2 cup non-stick muffin tins and bake for 15 minutes or until the muffins are cooked in the middle (poke with a toothpick and if comes out clean they are done). Cool them in the tins.