Place walnuts and 2 tbsp of the flour in food processor. Process until walnuts are finely chopped. Add remaining flour, sugar and salt and combine. Pulse until blended. Add butter and pulse just until small crumbs form. Add egg and vanilla and process just until dough holds together. Remove from processor and shape into a flat disc. Wrap and chill for at least 30 minutes or for up to 2 days.
Roll out pastry about 1/8 inch thick. Fit into an 11 inch tart pan with removable bottom or 9 inch pie plate; patch any cracks to seal. Trim and flute edges, as desired.
Chill pastry for 30 minutes. Re-roll any scraps and cut out shapes with a cookie cutter for garnish, if desired. Place on baking sheet and chill for 30 minutes.
Preheat oven to 375°F
Prick pastry with a fork. Bake on bottom rack of oven for about 20 minutes or until pastry is golden. Cool completely. If using, bake pastry cutouts for 5 to 8 minutes or until golden. Set aside.
Heat 2 cups of the milk in heavy saucepan, stirring over medium heat just until steaming and bubbles form around edge. Whisk sugar, cornstarch, remaining milk and egg yolks in a medium bowl. Slowly drizzle in about one third of the hot milk, whisking constantly. Whisk back into saucepan. Stir over medium heat until thickened, 5 to 10 minutes. Remove from heat and stir in butter and vanilla. Pour into bowl and place plastic wrap directly on the surface. Cool slightly.
Pour filling into crust. Cover with plastic wrap. Refrigerate until set. Whip cream with sugar and pipe into cooled filling. Garnish with pastry cutouts and/or Sweet Candied Walnut halves.