Melt 6 ounces chopped white chocolate with 1/2 cup whipping cream in a medium metal bowl set over a saucepan of gently simmering water. Remove the bowl from water and let the chocolate cool to room temperature.
In another medium bowl, whisk 1/2 cup whipping cream, one 8-ounce container mascarpone cheese, and 1 teaspoon vanilla extract until blended and smooth. Fold melted chocolate into mascarpone. Serve the dip with fresh fruit. Strawberries and cherries are terrific with it, but peach and nectarine slices would also be fun.