In a medium mixing bowl, combine brown sugar, butter, flour, cinnamon, 1/4 tsp salt, and 1/2 tsp vanilla extract. Stir until combined.
Roll out both pie crusts until they're about an inch or so larger than their original size. It should be about 11" in diameter.
With a 3" biscuit cutter, lightly score about 8 circles into the bottom crust. Using a fork, score the bottoms of each circle.
Spoon about a tablespoon of the brown sugar mixture into the center of each circle, leaving about 1/2" edge.
Beat the egg with a few tablespoons of water. Using a pastry brush, paint each bare edge with a bit of the egg wash.
Gently lay the second pie crust over top and cut out each of the hand pies with the biscuit cutter.
Press the tines of a fork around the edges of the hand pies to seal them and poke holes in the tops.
Arrange on a baking sheet with parchment paper and chill for half an hour.
While the pies are chilling, preheat the oven to 400 degrees F.
Bake hand pies for about 20 minutes or until the tops are nice and golden brown.
While the pies are baking, making the icing by combining the powdered sugar, remaining salt and vanilla extract, and heavy cream. Add enough heavy cream to make a runny icing, about 4 - 5 tablespoons.
Once pies are done, remove from the oven and carefully transfer to a wire rack and let cool a bit. Drizzle over icing and sprinkles (if desired) and cool once more.