Preheat oven to 325°F (160°C). Place paper liners in wells of Muffin Pan. Whisk flour and baking powder in Small Batter Bowl. Combine milk and vanilla in small bowl. In a 2qt mixing bowl, beat egg whites on high speed of electric mixer until stiff peaks form. In 4qt mixing bowl beat butter and granulated sugar on medium-high speed until fluffy; beat in sour cream until smooth. On medium speed, add flour mixture in three additions, alternating with milk mixture in two additions, mixing until smooth. Whisk in one-third of the egg whites. Fold in remaining egg whites using whisk. Using a large scoop, divide batter evenly among liners.
Bake 17-20 minutes or until wooden pick inserted in centers comes out clean. Remove pan from oven to a cooling rack. Immediately snip a 1/2-inch-deep (1-cm) “x” into top of each cupcake using kitchen shears. Press one truffle into each cupcake until flush with top of cupcake. Remove cupcakes from pan. Cool 30 minutes or to room temperature. (Truffles will melt into cupcakes.).
Meanwhile, for frosting, place white chocolate in microwave safe bowl, uncovered, on HIGH 1-1 1/2 minutes or until chocolate is mostly melted, stirring every 30 seconds. Let stand 3 minutes or until slightly cooled. In a large mixing bowl, combine melted chocolate, cream cheese, butter and vanilla; using clean beaters, beat on medium speed until well blended. Slowly add powdered sugar; beat well.
Place frosting in a piping bag fitted with open star tip. Pipe frosting evenly around tops of cupcakes. Garnish cupcakes with chocolate curls.