Line a cookie sheet with parchment paper or a silicone baking mat.
Roll out both of your pre-made pie dough circles into rectangles. (You will have to do a little bit of trimming)
Cut out 12 even rectangles.
Lay the rectangles on the cookies sheet.
In a bowl, mix your egg and milk for the egg wash. Use a pastry brush to brush each piece of dough with the egg mixture.
In a medium sized bowl, mix together your brown sugar, cinnamon and flour for the filling.
Scoop about 1 tablespoon of filling on 6 of the dough rectangles.
Spread around the dough, leaving a 1/4 of an inch around the rim clear.
Cover the 6 dough rectangles with the top dough pieces. Use a fork to close the seams around the edges.
Brush the tops with egg wash.
Poke small holes in the top with a fork to allow flaking.
Bake for 18 minutes.
Allow to cool for at least 15-20 minutes.
In another bowl, mix together your powdered sugar, milk, vanilla, and cinnamon for the icing. (You want it to be a thicker consistency)
Spread over the tops of the Pop Tarts when cooled. Let the icing sit for at least 30 minutes. (They are the BEST the next day when everything has set and hardened)