Line a tray that you can fit in your refrigerator (or baking sheet) with a sheet of parchment.
Press the dough into a 4 inch square, then dust both sides lightly with flour. Set it on a lightly floured pastry mat and roll into a rectangle that's about 8 x 9 ½ inches. This doesn't need to be a perfect rectangle because there will be lots of trimming. The dough should be about ⅛ of an inch thick.
Using a ruler and a knife or pizza cutter, cut the rectangle into 6 rectangles that are about 4 by a little over 3 inches each. Just make one cut through the 8 inch side and 3 cuts through the 9 ½ inch side.
Using a flat spatula, scoop up a rectangle of dough and transfer to the baking sheet. Repeat with two more, being careful not to mess up the rectangles too much.
Spoon about a tablespoon of the filling into the center of each rectangle, leaving about ¾ of an inch perimeter. Working carefully, one tart at a time, lay a second rectangle over the filling. Use your index finger to seal in the filling and press down the edges. Try to make the edges about ¾ of an inch. They'll be much wider than a normal Pop Tart, but you are going to trim after baking.
Put the shaped and filled pastries in the refrigerator for 30 minutes and preheat.
Preheat oven to 350 degrees F.
Transfer the parchment with the tarts to a baking tray. If you used a tray that would fit in the refrigerator and the oven, you can skip this.
Bake on center rack at 350 degrees F for 16 minutes or until the edges are lightly browned and the centers appear set. Remove from the oven and use a pizza cutter to carefully trim the edges to make neat rectangles. Transfer the parchment with the tarts to a cool surface while you make the icing.