Preheat the oven to 350 F and place a rack in the center of the oven. Once chilled, take the dough out of the freezer. Unwrap the dough and cut it in half, rewrap one half and place it in the fridge to stay cold.
Place the one piece of dough between two sheets of parchment paper and carefully use a rolling pin to roll it out to approximately 5.5" x 8".
Use a bench scraper or knife to trim any areas that are going out of shape and add them back to any where that isn't quite hitting the shape. Take your time with this - the neater and more precise you are here, the better your bars will look.
Line a baking sheet with a piece of parchment. Peel away the top layer of parchment from the dough you've just rolled out. Using the second piece of parchment, flip the dough over onto the parchment lined baking sheet and peel away the remaining parchment.
Repeat with rolling out the second piece of shortbread for the tops of the bars.
Spoon the chilled jam overtop and spread it all the way to the edges.
Using the same method, peel away one sheet of parchment from the second piece of dough and use the remaining parchment to flip the dough over onto the jam. Peel away the remaining parchment and you should now have stack of dough, jam, dough.
Press down on the top layer of dough slightly but there is no need to seel the edges.
Bake the bars for 35-40 minutes until the top layer of dough look dry and the edges have started to brown.
Once baked take the tray out of the oven and leave it to cool to room temperature before glazing.