To make pastry, sift flour into a large bowl.
Using a pastry blender or 2 knives, cut in butter until evenly distributed and mixture resembles breadcrumbs.
With a fork, lightly mix in sugar and egg to make a dough.
Press into a ball and wrap in foil or plastic wrap.
Refrigerate 30 minutes.
Preheat oven to 375°F (190 C).
On a floured surface, roll out dough to fit a 9-inch (23-cm) flan tin with a removable bottom.
Place dough in tin without stretching.
Bake blind, 20 minutes or until golden.
Cool.
To make filling, combine 1/3 cup (100 ml) milk, cornstarch, sugar and egg yolks in a medium bowl.
Bring remaining milk to a boil in a medium saucepan.
Stir hot milk into cornstarch mixture.
Return to saucepan and bring to a boil, stirring constantly until thickened.
Let custard cool, stirring frequently to prevent a skin forming.
Beat egg whites until stiff; fold into cooled custard.
Place half the filling in prebaked pastry shell.
Reserve 12 grapes for decoration; arrange remainder on top of custard filling.
Cover evenly with remaining filling.
Refrigerate until set then remove from tin.
Whip cream.
Put in a pastry bag fitted with a fluted nozzle.
Pipe rosettes onto flan; decorate with reserved grapes.