To make jam filling: Mix the jam with the cornstarch/water in a small saucepan. 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 pastry tarts.
Make the dough: Whisk together the flour, sugar, and salt. Work in the butter with your fingers, pastry blender or food processor until pea-sized lumps of butter are still visible, and the mixture holds together when you squeeze it. If you've used a food processor, transfer the mixture to a large bowl. Whisk the first egg and milk together and stir them into the dough, mixing just until everything is cohesive, kneading briefly on a well-floured counter if necessary.
Divide the dough in half, shape each half into a smooth rectangle, about 3.5 inches. You can roll this out immediately or wrap each half in plastic and refrigerate for up to 2 days.
Assemble the tarts: If the dough has been chilled, remove it from the refrigerator and allow it to soften and become workable, about 15 to 30 minutes. Place one piece on a lightly floured work surface, and roll it into a rectangle about 1/8 inch thick, large enough that you can trim it to an even 9 x 12 inches. Repeat with the second piece of dough. Cut each piece of dough into thirds - you\'ll form nine 3 x 4 inch rectangles.
Beat the additional egg and brush it over the entire surface of the first dough. This will be the inside of the tart; the egg is to help glue the lid on. Place a heaping tablespoon of filling into the center of each rectangle, keeping a bare 1/2-inch perimeter around it. Place a second rectangle of dough atop the first, using your fingertips to press firmly around the pocket of filling, sealing the dough well on all sides. Press the tines of a fork all around the edge of the rectangle. Repeat with remaining tarts.
Gently place the tarts on a lightly greased or parchment-lined baking sheet. Prick the top of each tart multiple times with a fork. Chill for 30 minutes. Bake in preheated 350 degree oven for 20 to 25 minutes, until they’re a light golden brown. Cool in pan on rack.