In a medium sauce pan combine milk, cornstarch, sugar and egg yolk. Mix together and cook over medium low heat. Stir constantly and don't let it boil. It will thicken up.
Remove from heat and add in vanilla, butter and white chocolate. Stir until chocolate is melted.
Cover with plastic and chill for 3-4 hours or overnight.
In a small bowl combine gelatin and water. Let it stand for five minutes, then microwave for 10 seconds to liquefy it. Cool to room temperature.
In a medium bowl, combine heavy cream, powdered sugar and vanilla. Beat until it thickens. Gradually add in cooled gelatin mixture and beat until medium peaks form.
Fold the whipped cream mixture into the white chocolate mixture.
Cover and chill for one hour.
Notes
Recipe makes 3 cupsThe filling can easily be made the day before. Just keep it refrigerated until ready to use. I recommend making the filling the night before so that it has a chance to thoroughly chill and set up.