In the bowl of a food processor fitted with the blade attachment, combine all-purpose flour, unsalted butter (cold and cut into small pieces), cornstarch, sugar, and salt. Pulse until mixture resembles coarse cornmeal. In a small bowl whisk together egg yolk, cold water, and vanilla extract until well combined. While pulsing, slowly pour egg mixture into flour mixture. Pulse until just moistened; turn onto plastic wrap. Gently knead until any dry patches are incorporated. Wrap and refrigerate at least 20 minutes.
Preheat oven to 350 degrees F. Crumble 2/3 of the shortbread dough over the surface of a 9-inch deep-dish pie plate. Gently press dough evenly onto bottom and sides; trim excess. Prick bottom and sides of pastry with a fork. Chill 45 minutes. Line pan with parchment; fill to the top with dried beans or pie weights. Bake 15 minutes. Carefully remove parchment and weights; loosely cover with foil. Bake 5 to 8 minutes more or until dough begins to brown. Cool on a wire rack.
In a large mixing bowl beat together sugar, butter, and a pinch of salt until light and fluffy. Add flour and cornmeal; mix until just incorporated. Add eggs, one at a time, until incorporated, scraping down sides of bowl between additions. Add 2 drops orange food coloring, if desired.
In a small bowl whisk together creme fraiche, clementine and lemon juices and peels, and vanilla; beat into butter and sugar mixture. Scrape down sides; mix until incorporated. (It may look curdled. This is OK.) Pour into prepared crust. Bake 35 to 40 minutes or until pie is just set but still has a very slight wiggle in center. Cool on a wire rack
Prepare gold cookie medallions (see Golden Dress-Up, below) and Quick Candied Clementines. Serve on pie.
Roll the remaining crust dough to 1/8" thick between two sheets of waxed paper or parchment paper. Place on a baking sheet and chill 5 minutes or until firm. Using an embossed maple cookie stamp, press gently on the dough, and using a round cutter, stamp out medallion shapes. Place the medallions on a parchment lined baking sheet and bake at 350 degrees F for 5 to 10 minutes or until just golden brown. Allow to cool completely on a wire rack. Spray each medallion with Wilton gold spray and allow the spray to dry.
Quick Candied Clementines
Cut clementines into paper-thin slices; brush generously with melted orange marmalade; let dry completely.