Make the shortbread crust: Combine the butter, sugar, salt, vanilla and egg in the bowl of a stand mixer. Mix on low, then medium speed with the paddle attachment until well combined. Add the flour in two additions. Starting on low and increasing to medium speed. Scrape down sides and paddle as needed.
Place the dough on a floured work surface. Pat into a disk. Cover in plastic wrap and refrigerate for 30 minutes.
Make the filling: combine all the filling ingredients in a bowl. Mix well.
Pre heat oven to 350 degrees fahrenheit, with the rack in the middle.
Spray a 7 3/4 x 1 inch round tart pan with removable bottom, with cooking spray, Roll out 2/3 of the dough, until its a bit larger than the tart pan. Gently move the dough onto the pan and press into the edges and the bottom. With a fork, pierce some holes into the bottom of the dough for venting.
Pour the filling into the tart pan. Place the tart pan onto a baking sheet, this will avoid a mess in your oven.
Lightly wet the edges of the dough with water. Roll out the rest of the dough and cut into stars or any shape you wish. Place the shapes onto the top of the filling. You will have enough dough to have a full sheet, should you wish to cover the entire tart.
Be sure to pierce holes on the top if you are going to cover the entire pan. Brush dough with the egg wash. Bake for 50 - 55 minutes.
*Caster sugar is a very fine sugar. You can either find it at your local market or make it yourself. All you need to do is pulse granulated sugar in your blender or food processor for a few seconds.
*This dough can be made ahead and frozen. Prepare, wrap tightly in plastic wrap, then in a Ziplock bag. Remove from the freezer at least an hour before use. Can be frozen up to 3 months.
*Use within 3-5 days if refrigerating.
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