In a medium bowl, blend the powdered sugar, milk, ½ tablespoon of cinnamon, and ½ tablespoon of vanilla extract. Set this mixture aside.

In a separate large bowl, combine the brown sugar, butter, remaining cinnamon, and vanilla extract until well mixed.

On a lightly floured surface, roll out the pie crust to a thickness of ¼ inch. Cut the crust in half with a pizza cutter, setting one half aside and rolling out the other half.

Place one-half of the crust onto a lined baking sheet, allowing one edge to hang over the side. Spread half of the cinnamon glaze evenly on the bottom half of the crust.

Fold the top half of the crust over the brown sugar filling, pressing the edges together to seal. As you crimp or pinch the edges shut, ensure any air trapped inside the tart is released.

Beat an egg in a small bowl or cup to create an egg wash. Use a brush to apply the egg wash to the top of the sealed tart.

With a toothpick, poke several holes into the top of the tart to allow steam to escape during baking.

Bake the tart for approximately 8 minutes on the middle rack until it turns golden brown.

Allow the tart to cool to room temperature, then drizzle the icing over the top. Cut the tart into desired portion sizes and enjoy!