Instructions

Make the crust

1. Preheat oven to 350°F. Add the flour, powdered sugar, and salt to a small bowl and whisk together. Melt the butter in a small saucepan (or in the microwave), add the vanilla extract and orange zest to the butter and stir, then pour into the dry ingredients and stir together until thoroughly moistened. The crust will look wet. Let cool for a few minutes until it firms up slightly.
2. Press the dough evenly along the bottom and sides into a 9-inch tart pan with a removable bottom. Place the tart pan on a baking sheet and bake until the crust is golden brown, about 20 minutes.

Make the cranberry curd

1. Heat the cranberries and orange juice in a medium saucepan over medium-high heat until the cranberries split. Strain through a food mill or fine mesh sieve and discard the skins.  [Edit: this mixture is thick and takes some muscle to press it through a fine mesh sieve. It's easier with a food mill, but if you're struggling with this, put the mixture in a blender and then pass it through a sieve.]
2. Return the strained cranberry mixture to the saucepan and add the sugar, eggs, egg yolks, butter, vanilla, and orange liqueur (if using) to the pan and give everything a good stir. Heat gently over medium-low heat, stirring constantly, until the mixture thickens and registers 170°F on an instant read thermometer (this usually takes about 8-9 minutes). Immediately strain through a fine mesh sieve into a bowl (ideally with a pouring spout) and then pour into the prepared crust. Bake (still at 350°F) until the curd is set (it should jiggle but not slosh), about 10 minutes.
3. Let cool at room temperature for about 1 hour, then transfer to the refrigerator to chill. You can serve this chilled or at room temperature.

Make the Italian meringue topping (totally optional)

1. Add egg whites and cream of tartar to the bowl of a stand mixer fitted with a whisk attachment. Heat sugar and water in a small saucepan over medium high heat. When the sugar mixture registers about 220°F turn on the mixer and begin beating the egg whites on medium speed. When the sugar mixture registers 240°F remove from heat and carefully pour into the mixer over the egg whites. Increase the mixer speed to high and beat until the egg whites form stiff peaks. Transfer to a piping bag (or zip top bag) and pipe onto cooled tart as desired. If you want, torch the meringue with a kitchen torch. (But since it is cooked, this is totally optional).