Preheat the oven to 350°F.
If you have chilled the dough for longer than an hour, let it come to room temperature for about 15-20 minutes.
Place one disc of your dough on to a lightly floured surface and roll into a rectangle that is around 1/8″ thick. You will want your rectangle large enough to trim to 9×12″. Cut your rectangle in half horizontally (at the 4.5″ mark) then vertically every 4 inches. Making 8 - 4.5"×3″ squares.
Place the rectangles onto a prepared baking sheet (with aluminum foil). Space them out evenly on the sheet tray.
Prepare the egg wash by whisking the egg in a small bowl, and brush each rectangle with a light egg wash.
For the brown sugar filling, mix the ingredients in a small bowl. Take your filling of choice, and place about 1 tablespoon of filling in the middle of each rectangle.
Next, prepare the tops. Place the second disc of your dough on to a lightly floured surface and roll into a rectangle that is around 1/8″ thick. You will want your rectangle large enough to trim to 9×12″. Cut your rectangle in half horizontally (at the 4.5″ mark) then vertically every 4 inches. Making 8 - 4.5"×3″ squares.
Place one rectangle on top of each prepared and filled rectangle, to "top" off the pop tart. Press the edges with your fingers lightly to seal the filling in. Using a fork, you can seal the edges with the edge of the fork to create the lined look. Poke each pop tart 3-4 times with the edge of the fork to let steam release as it bakes.
Brush the top of each pop tart with the egg wash.
Place the Pop Tarts in the oven and bake for 20-25 minutes until they are golden. Let cool slightly and transfer to a wire rack to cool completely.