Preheat oven to 350° degrees F. Line 2 baking sheets with parchment paper and set aside.
On a clean, flat work surface, roll out one pie crust sheet. Using a sharp knife or a pizza cutter, trim off the round sides of the dough creating more of a square shape. Within that square, cut out 6 small rectangles. Set the rectangle dough pieces 2" apart on one of the prepared baking sheets. Repeat with remaining pie crusts, working with one crust at a time. Try to keep the pie dough rectangles the same or a very similar size in shape.
In a small bowl, lightly beat the egg and set aside. In a medium bowl, whisk together the brown sugar, flour, and 1 Tablespoon of cinnamon until blended.
Brush half (12) of the rectangles with the egg wash and top with a heaping Tablespoonful of the brown sugar and cinnamon mixture, taking care to keep the filling toward the center of the Pop Tart and keep a little bit of the edges clean. Repeat with remaining Pop-Tarts. Take a similarly sized plain rectangle of dough and lay it on top of the filling, pressing the edges to seal together with the bottom half. Use a fork to crimp the edges closed. Brush the tops with the remaining egg wash.
Bake for 20-22 minutes or until the Pop-Tarts are golden in color. Cool for about 15 minutes on the pans. Meanwhile, make your glaze: in a medium bowl, whisk together the confectioners' sugar, milk, vanilla and remaining cinnamon until smooth. Spoon Tablespoonfuls of the glaze on top of the still-warm Pop-Tarts. Allow the glaze to set before serving or storing.