In a small saucepan bring the 1 cup berries and the water to boiling; reduce heat. Simmer, covered, for 5 minutes. Press the mixture through a fine-mesh sieve; discard any solids. Return juice to saucepan. Stir in the 1/2 cup sugar. Bring mixture to boiling, stirring to dissolve sugar. Boil gently, uncovered, for 2 minutes. Transfer to bowl. Cool.
Using a fork, poke holes in tops of yellow cupcakes. Slowly spoon berry mixture over cupcakes. Set aside.
In a medium mixing bowl beat whipping cream, sour cream, and the 2 tablespoons sugar with an electric mixer on medium speed just until thickened and soft peaks form (tips curl). If desired, place cream mixture in a pastry bag fitted with a large star tip. Generously spoon mixture onto tops of cupcakes. Serve immediately or chill in the refrigerator for up to 4 hours.
Make white chocolate shavings or curls by pulling a vegetable peeler along edge of the white chocolate. Set aside.
To serve, garnish tops of cupcakes with the 2-1/2 cups berries. Sprinkle with white chocolate. Makes 12 (2-1/2 inch) cupcakes.