Chocolate and Orange Short Crust
Combine the flour, cocoa powder, powdered sugar, butter, and salt in bowl of a stand mixer with paddle. Mix on speeds 1-2 until the mixture comes together. Add the vanilla and orange zest.
Press the dough evenly into the tart pan and the fluted edges. Chill until firm.
Preheat oven 375 degrees F.
Line the pan with foil and bake for 15 minutes. The baked dough may puff up, press down with the back of a spoon or spatula. Let cool.
Blood Orange Curd
Combine the juices and zest; reserve.
Whisk the yolks, whole eggs, and sugar in a non-reactive bowl that will sit over a pot of simmering water to create a bain marie or double boiler.
Continue to whisk the mixture over simmering water until thick and lighter in color. Add 1/3 of the fruit juice. Whisk until it begins to thicken, then add the remaining 2/3 juice continuing to whisk constantly. The curd must be quite thick and have absorbed all of the foam; the whisk will leave a trail at the bottom of the bowl. Off the heat, whisk in the butter until blended.
Pour into the baked crust or chill the curd in the refrigerator and finish the tart the following day. The curd holds overnight covered; fill the crust several hours before serving.
Creme Fraiche Topping
Combine the creme fraiche, lemon zest, and sugar. Whip until light and fluffy. Spread over the top of the tart before serving.