In a saucepan, combine sugar and cornstarch; gradually whisk in milk until smooth. Bring to a boil over medium heat. Cook and stir for 2 minutes. Remove from the heat. Stir a small amount of hot mixture into egg yolks; return all to saucepan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes.
Remove from the heat; stir in white chocolate and vanilla. Let stand until chocolate is melted, stirring occasionally. Pour into pastry shell. Refrigerate for at least 3 hours.
For holly leaf garnish, melt 1/2 oz. white chocolate; tint with red food coloring. Place in a pastry or plastic bag. Pipe 24 small berries on waxed paper; let stand until hardened. Melt remaining white chocolate; tint with green food coloring. Spread into a rectangle on waxed paper; let stand at room temperature until harden. Using a 1-1/2-in. holly leaf cookie cutter, cut 18 leaf shapes.
Just before serving, beat cream and confectioners' sugar until soft peaks form; spread over pie. Garnish with holly leaves and berries. Store in the refrigerator.
Editor's Note: For a simple garnish, use spearmint leaves and red-hot candies.