Start with the tart crust: Place the flour, sugar, and salt in the bowl of a food processor or large mixing bowl. Pulse or cut the butter using a pastry cutter until the mixture resembles coarse sand. Add the egg yolk, cream, and vanilla, pulsing or stirring until it gathers itself into a ball. Wrap the dough tightly in plastic wrap and chill for 1 hour in the refrigerator.
Once chilled, preheat the oven to 325ºF.
On a lightly floured surface, roll the dough until it's 1-2 inches wider than the tart pan. Carefully press the dough into the tart pan, trimming away the excess. Bake for 18-20 minutes until the edges begin to turn golden brown.
While the crust bakes, begin the filling: Add the cranberry sauce, eggs, egg yolks, butter, and salt in a large heat-safe bowl. Place over a pot of simmering water to create a double boiler. Whisk until the mixture begins to thicken and reaches 170ºF– this will take about 10-12 minutes.
Pour the filling into the par-baked crust and smooth into an even layer. Bake for 16-20 minutes until the edges are set and the crust is golden brown. Allow to cool completely before topping with meringue.
Make the meringue: Place the egg whites and cream of tartar in a large bowl. Using a hand or stand mixer on high speed, fitted with the whisk attachment, whip until foamy– about 10 minutes.
Reduce the speed to medium and slowly stream in the sugar. Once the sugar is combined, increase the speed to medium-high until stiff peaks form, about 1-3 minutes.
Spread or pipe the meringue on top of the cooled tart. Toast the meringue with a torch or place under the broiler (keep a very close eye on it). Add desired toppings like fresh cranberries. Leftovers stay fresh in the refrigerator for up to 3 days.