For the filling and tarts: Heat 1/2 cup of the milk in a small saucepan over medium heat. Meanwhile, whisk the remaining 1/4 cup milk with the maple sugar, egg yolk, cinnamon, vanilla and cornstarch in a medium bowl. Slowly pour the hot milk into the bowl while whisking, and then pour all the mixture back into the saucepan. Bring to a simmer and cook until thick, about 1 minute. Strain through a fine mesh strainer into a bowl and stir in the butter. Cool to room temperature, stirring occasionally.
Preheat the oven to 400 degrees F. Line a rimmed baking sheet with parchment paper.
Dust your counter with flour and unroll one of the pie crusts. Square off the edges to a 7-by-8-inch rectangle, and then cut it onto 4 equal rectangles (3 1/2-by-4-inches). Repeat with the other pie crust.
Lay out 4 of the rectangles and spoon 2 tablespoons of the custard filling onto the center of each. Spread the filling slightly, leaving a 3/4-inch boarder. Whisk the whole egg in a small bowl and brush the edges of the pastry rectangles with the egg wash. Top with the remaining rectangles of dough and press the edges together to seal. Use a fork to decoratively crimp the edges of each tart.
Arrange the pastries on the prepared baking sheet and bake until golden brown, about 16 minutes.
For the glaze: While the tarts bake, whisk together the confectioners' sugar and maple syrup in medium bowl. Add milk, about 1/2 teaspoon at a time, until the glaze is a thick but spreadable consistency.
When the tarts are still hot, spoon the glaze onto the tops. Gently spread to coat with a small offset spatula and sprinkle immediately with the sprinkles, working quickly so that the glaze doesn't set before the sprinkles can stick.