In a medium bowl, mix together the softened butter, flour, powdered sugar, vanilla, and salt until completely combined. Dump the crumb-like mixture into the 9-inch tart pan. Start with the edge, and firmly press the crumbs together to form about 1/2 inch thick edge. Pierce the bottom with a fork to create a few vent holes.
Bake for 17-20 minutes, or until golden brown and firm. Set aside to cool completely. In a medium saucepan, simmer the cranberries on medium heat with 1/2 cup of water until cranberries have burst, about 10-15 minutes. Unless the pot is very deep, you may want to place a lid on top.
Place mixture into a high speed mixer, and blend until smooth. Add the quarter cup cranberry juice to bring the mixture to the desired texture. It should be the consistency of a thin jam.
Strain through a sieve to catch any pieces of cranberry skin that may be left. You may need to push the mixture through the sieve with a spatula. This step is important to create a smooth texture in the curd. Place mixture back into the saucepan.
Stir the brown sugar and egg yolks into the cranberry mixture in the saucepan, and continue to heat on medium heat, whisking constantly.. In a small bowl, stir together lemon juice and cornstarch until smooth. Add to the saucepan, and whisk until incorporated. Cook until thickened, whisking constantly. Remove from heat, and whisk in the butter, one tablespoon at a time until smooth, and then the vanilla. Mix until everything is smooth and incorporated, and pour into the cooled tart shell.
Place in the fridge and chill for 4 hours, or overnight. Top with whipped cream or sugared cranberries.
Store in an airtight container in the fridge for a few days.