Pre-heat the oven to 375 degrees. Melt 1/2 cup of the butter and set aside. Have a sheet pan at the ready.
Unroll 2 of the pie crusts onto a piece of parchment paper that is the size of a sheet pan (about 12 x 17 inches), overlapping them so that they are the total length of the parchment paper. Use a rolling pin to roll them into one smooth crust that is also the width of your baking sheet. You can take any excess dough to patch in where needed.
Use a pizza cutter or small knife to trim the dough into a neat rectangle. Roll the dough and parchment paper onto your rolling pin and unroll, crust side up, onto your baking sheet.
Make the brown sugar filling:
Use a fork to mix the melted butter, brown sugar, cinnamon and flour in a small bowl. Pour onto your crust and use the fork to spread out evenly, leaving a one-inch border on all four sides. Smooth the filling and place the pan in the fridge while you prepare the top crust.
Make the top crust by rolling and trimming the remaining two pie crusts as you did the first two. Roll onto your rolling pin and gently unroll - with the parchment side up - onto your filled crust. Remove and discard the parchment paper.
Use your fingers to seal the edges of the pie, then go around the edges again with a fork to crimp. Poke holes all over the center of the pie with a toothpick.
Brush the remaining 3 tablespoons of butter all over the top of the pie and bake for 20 - 25 minutes, or till golden brown. Cool completely in the pan.
Make the glaze:
Whisk all of the ingredients in a small bowl till thick but pourable. Add more milk if needed, a little at a time.
Pour the glaze over the center of the cooled pie, and use a small angled spatula to spread evenly, leaving a small border around the edges. Let the glaze set before cutting into squares to serve.
Notes
You can store the slices covered, at room temperature, for up to 3 days or freeze for longer storage.