Citrus Shortbread Crust
Lemon Basil Filling
Preheat oven to 300 to 325 degrees Fahrenheit. Cream butter using a kitchen mixer with a paccle attachment. Gradually add sugar until combined. Slowly add flour, salt and zests and mix at medium high until combined. Scrape dough from the sides of the bowl to form a ball.
Roll dough out to 1/8-inch thickness. Line four 3-inch tart tins with dough. Dough should come up the sides at least 1-inch. Pierce dough all over with a fork. Blind bake 8 to 10 minutes. There should be little to no colour. Cool on a rack. Remove tart shells from the tins.
Preheat oven to 250 degrees Fahrenheit.
Combine all ingredients in the top of a double boiler or in a large bowl placed over a pot of simmering water. Whisk continually until mix is heated through to prevent eggs from curdling. Continue to cook over double boiler, whisking every few minutes. Cook until mixture reaches a custard like consistency (coats the back of a spoon), for about 15 to 20 minutes. Remove from heat and strain through a fine mesh strainer into a clean bowl and let cool for 10 minutes.
Place tart shells on a baking sheet and fill with custard. Bake until custard sets, about 10 minutes. Remove from oven and allow to cool before handling.