Preheat oven to 350 degrees F. In a medium mixing bowl sift cocoa, flour, baking soda and salt. In a large mixing bowl combine stout, melted butter, and vanilla. Beat in eggs, one at a time then mix in yogurt or sour cream. Add dry ingredients to wet a little at a time. Pour evenly into muffin tin lined with cupcake liners. Bake for 15 to 22 min until middle is set. Allow to cool. Make icing. Top with Lindt chocolate bomb after icing.