Preheat oven to 350 F.
Lightly dust a work surface with flour.
Working one sheet at a time, roll out the pie crusts to about a 1/8" thickness and cut into 3" x 4" rectangles. You should get 6 rectangles out of each dough roll.
Place 12 of the dough rectangles onto baking sheets. These will be the bottoms of the tarts.
Beat the egg with 1 tablespoon water and brush onto the crusts.
Make the filling by combining all ingredients and place 1 heaping tablespoon onto the center of each tart bottom. Do not get the filing within 1/4" of the edges of the dough.
Place the remaining dough rectangles on top of the bottoms and press down along the edges with your fingers to seal. Use a fork to then crimp the edges.
Brush tops with egg wash.
Bake 20-25 minutes or until golden brown.
Remove from oven and let cool completely.
Make the icing by combining all the of ingredients, beating with a fork. Add more milk if the mixture is too thick, but don't add too much or it'll be too thin.
Brush icing on top of tarts.
To serve, reheat in a toaster or toaster oven.