Heat oven to 350°F. In small bowl, blend egg yolk with 1 tablespoon of the orange juice. Place in food processor bowl with metal blade.
Add flour and all remaining crust ingredients except remaining orange juice; process until mixture is crumbly. With food processor running, add enough orange juice to form dough. Press dough in bottom and up sides of ungreased 10-inch tart pan with removable bottom. If dough is sticky, flour hands.
Bake at 350°F. for 17 to 22 minutes or until edges are light golden brown. Cool 1 hour or until completely cooled.
Meanwhile, in medium saucepan, combine all filling ingredients except butter; beat with wire whisk until well mixed. Cook over medium-low heat until mixture begins to bubble and is very thick, stirring constantly. Remove from heat. Stir in 1/2 cup butter until melted. If desired, strain filling. Pour into nonmetal bowl. Cover; refrigerate 1 hour or until completely cooled.
Pour cooled filling into cooled crust. Refrigerate at least 2 hours to set.
Beat whipping cream with powdered sugar until soft peaks form. Pipe around outside edge of tart. Garnish with edible flowers.