Preheat the oven to 350°F and line a large baking sheet with parchment paper or a silicone baking liner.
Microwave frozen sweet potatoes (in steamable bag) according to package directions, or until very soft.
Transfer cooked sweet potatoes to a large bowl and purée with an immersion blender or mash with a potato masher.
Add brown sugar, vanilla, and spices and whisk until smooth. Set aside.
On a lightly-floured work surface, roll out pie crust to 1/8th-inch thick. Using a pizza cutter or sharp knife, cut edges to form a rectangle.
Cut dough in half end-to-end, then into fourths across to make 8 rectangles.
Repeat with second pie crust, using previously formed rectangles as a guide, to form 16 rectangles of the same size and shape. Set 8 of the rectangles aside.
Spread filling in center of remaining pastry rectangles by heaping tablespoons, leaving ½-inch of pastry exposed all around.
Brush exposed pastry with egg wash. Top with reserved pastry rectangles and crimp edges with a fork. Brush tops of pastry with more egg wash and poke with a toothpick to create vent holes.
Bake in preheated oven for 25 minutes or until golden. Allow pastries to cool before icing.
To make cinnamon brown butter icing, whisk together all icing ingredients until smooth. Add more milk if needed to achieve thinner consistency. Spread over cooled pastries.
Sweet Potato: As I mention in the ingredients above, you can substitute the flesh of 2 medium sweet potatoes for the frozen sweet potato chunks. You can also use 1 cup of your favorite prepared sweet potato pie filling -- just be sure to omit the remaining filling ingredients and follow the assembly instructions from there. Pie Crust: I used store-bought pie crust for this recipe to save time, but you can also use your favorite homemade pie crust for an even more delicious, flaky pastry!