Divide the pie dough into two flattened discs; wrap in plastic and chill.
Preheat the oven to 375 degrees F. Line two cookie sheets with parchment.
Working one disc at a time, roll out the dough into a 1/8-inch-thick rectangle on a floured work surface. Cut into 9 even rectangles, then transfer to the prepared cookie sheets and chill.
Remove one of the cookie sheets of portioned dough to a work surface. Whisk the egg and brush it evenly on the surface of the rectangles. Add 1 heaping tablespoon of the cookie dough to each rectangle. Cover with the rectangles on the second cookie sheet, pressing the edges with your fingers. With a fork, go over edges to seal and poke holes on the top to allow steam to exit. Place the assembled tarts back in the refrigerator for 30 minutes.
Bake for 16 minutes, until golden brown, rotating every 5 minutes for even cooking. Let cool for 5 to 7 minutes.
In a small bowl, mix together the powdered sugar, milk and vanilla; evenly ice each tart. Sprinkle with the mini chocolate chips.