Rinse your cranberries and add to a saucepan along with the brown sugar.
Bring up to a boil, stirring constantly to dissolve the sugar. Cook for a few minutes to allow the cranberries to pop and release their juices. Sprinkle in the Instant Clearjel and continue to cook until thickened to a loose jam consistancy. Remove from heat and let cool. (The filling will continue to thicken as it cools.)
Cream the soft butter and sugar until light and fluffy, about 3-4 minutes, scraping down the sides of the bowl as necessary. Beat in the vanilla and salt. Note: it's important that your butter be at room temperature. I like to leave it out overnight when I know I'll be baking.
Add the flour and continue beating, with your mixer on low, just until the mixture becomes crumbly and there are no dry patches of flour left.
Take 2/3 of the dough and pat it into the bottom of your tart pan. I like to scatter the dough in crumbles across the bottom, and then start to pat it down. It helps to flour your fingers, or use the bottom of a metal measuring cup to tamp it down. Take the time to get the dough evenly distributed. Note: you're not looking to go up the sides of the pan with the dough, just to make an even flat layer.
Spread the cranberry mixture evenly over the bottom crust.
Take the remaining dough and break it apart with your fingers to make coarse crumbles. Crumble evenly over the cranberry filling. It's ok if some of the cranberries show through.
Set the tart pan on a baking sheet, and bake in the preheated oven for about 35 minutes, or until the top is just starting to turn pale golden. I sometimes lay a sheet of foil loosely over the top for the second half of baking so it doesn't brown too much.
Let the tart cool for 15 minutes on a rack before releasing the bottom from the sides. Finish cooling on the rack.