Heat 1 inch of water in the bottom half of a double boiler over low heat. When the water is hot (do not allow to simmer), place white chocolate and 4 tablespoons of water in the top half of the double boiler. Using a rubber spatula, constantly stir the white chocolate and water until melted, about 4 to 5 minutes. Remove from heat and keep at room temperature until needed.

Place the heavy cream in a well-chilled bowl of an electric mixer fitted with a well-chilled balloon whip. Mix on high until stiff, about 1 minute. Remove the bowl from the mixer. Using a hand-held whisk, vigorously whisk 1/3 of the whipped cream into the melted white chocolate. Scrape down the bowl with a rubber spatula, and continue to whisk until smooth and thoroughly combined. Add the combined whipped cream and white chocolate to the remaining whipped cream and use a rubber spatula to fold together until smooth.

Transfer to a container and refrigerate for 2 to 3 hours before serving.

This is from Marcel Desaulniers' Death By Chocolate Cookbook.