In a small saucepan, melt the chocolate, milk and butter over low heat; stir until blended. Remove from the heat. Stir in vanilla. Transfer to a small bowl; cover and refrigerate until firm, about 1 hour. Roll into 24 1-inch balls; chill 1 hour longer.
In a large bowl, combine the cake mix, eggs, sour cream, oil, water and vanilla; beat on low speed for 30 seconds. Beat on medium for 2 minutes.
Fill paper-lined muffin cups one-third full. Drop a truffle into the center of each cupcake. Top with remaining batter. Bake at 350 degrees for 17–22 minutes or until a toothpick inserted in the center comes out clean.
Cool for 10 minutes before removing from pans to wire racks to cool completely. Top with whipped cream if desired.