Preheat oven to 400 degrees F. As it heats, slice pie dough into 1" by 2" rectangles and place on a parchment-lined baking sheet.
In a medium-sized bowl, combine pumpkin puree, brown sugar, vanilla, salt and cinnamon. Spoon about half a teaspoon full on one side of each dough rectangle. Dip your finger into the milk and trace the edge of each rectangle, then fold the dough in half, like you're closing a book, to create the mini tart.
Use a fork to crimp the edges of each tart. Brush the tops of each one with remaining milk. Place them on a parchment-lined baking sheet and bake for 12 to 14 minutes, or until lightly golden. Set aside to cool.
As the pastries cool, make the glaze: In another medium-sized bowl, add confectioners' sugar, water and vanilla. Whisk to combine. You may need to add a little more water or sugar to get it the consistency you like; I prefer it about as thick as pancake batter. Spread on cooled pastries and top with sprinkles before serving.
