In a medium-sized saucepan, melt the butter over medium heat. When the butter is melted, stir in the cocoa and yogurt, and bring the mixture to a boil., Turn the heat to very low, add the confectioners' sugar and vanilla, and beat until smooth., Working quickly, pour about half of the frosting atop one layer, spreading it around the sides. Center the second layer over the first, and pour on the remaining frosting, spreading it evenly to the edge and along the side of the cake.