Process the butter, cream cheese, and cream in an electric mixer fitted with a paddle or in a large food processor to thoroughly combine.
Add the flour and salt. Mix until just combined and the dough holds together in a ball. Turn the dough out onto a lightly floured surface. Divide the dough in half. Shape each half into a rough 3" x 5" rectangle, smoothing the edges. Wrap in plastic or waxed paper, and refrigerate for 30 minutes; this will relax the gluten and firm up the dough, making it easier to roll out. You can also refrigerate the dough for up to 2 days; let it rest for 15 minutes or so at room temperature before you roll it out.*
Mix the cornstarch with 2 tablespoons cold water, add the jam. Bring the mixture to a boil and simmer, stirring, for 2 minutes. Remove from the heat, and set aside to cool. Use to fill the tarts.
In a small bowl, whisk the egg with 1 tablespoon of cold water and a pinch of salt. Brush the egg wash on each of the rectangles. Spoon a tablespoon of filling into the center of each rectangle, leaving a ½-inch of space around the edges. Place your baking sheet with the tarts in the refrigerator while you roll out the tops.
Roll out and cut the second piece of dough the exact same way as you did the first. Prick the tops in multiple spots with a fork or a toothpick to allow steam to escape during baking.
One at a time, place a top rectangle of dough on the assembled ones. Using your fingers, press around the seams of the dough to make sure they are sealed. Press the tines of a fork around the edges of the rectangles. Refrigerate or freeze the pan with the pastries for at least 20 minutes (this will give you flakier tarts)
In a small bowl, whisk together the powdered sugar, cream, vanilla extract, and jam (if using). The icing should have a very thick, but still spreadable, consistency. If necessary, add cream, milk or water, a little at a time, until that consistency is reached. Spread a spoonful of icing over top of each pastry and allow to sit for at least 30 minutes so the icing can set (easier said than done; in my house every time I make these, at least one gets eaten before the icing is set)
*both the dough and the filled, unbaked tarts freeze incredibly well. To use the dough from frozen take it out of the freezer the night before and defrost in the refrigerator overnight. The filled and frozen tarts can be baked straight from frozen, you may need to add an additional 3-5 minutes to your baking time.