Step 2: Assemble the tarts
1. Preheat your oven to 350°F.
2. Divide the dough in half. Take the first half and roll it out into a large rectangle, 1/8" thick, on a lightly floured surface. You'll be cutting this into a 9"x12" rectangle, which you will then cut into eight 3"x4" rectangles.
3. Line up your eight dough rectangles to parchment paper on a baking pan. Repeat step 1 with the remainder of the dough.
4. Beat your extra egg and brush it over the surface of the first dough (on the parchment paper). This is the "inside" of the tart, and the egg is kind of like the "glue". Scoop a big tablespoon of jam in the middle of each rectangle. Leave about 1/2-inch perimeter around the tart.
5. Place the second rectangles on top of the first. Use your fingertips to press firmly around the tarts, sealing the dough on all sides. Press the outer rims of the tarts with a fork, and puncture the tops with your fork to let some steam escape while they're baking.
6. Bake for 30 minutes.