Preheat oven to 350°. Line 2 rimmed baking sheets with parchment paper.
On a lightly floured surface, unroll 1 sheet piecrust dough.
Using a rolling pin, gently roll dough into an 11-inch circle.
Using a 2 3⁄4-inch heart-shaped cutter, cut 20 shapes from dough, rerolling scraps if necessary.
Repeat procedure with remaining dough sheet.
Place hazelnut chocolate spread in a piping bag, and cut a 1⁄8-inch hole in tip of piping bag.
Spoon marshmallow crème into another piping bag and cut a 1⁄4-inch hole in tip of piping bag.
In a small bowl, whisk together egg and 1 teaspoon water to make an egg wash.
Using a small pastry brush, brush edges of 20 dough hearts lightly with egg wash.
Pipe hazelnut chocolate spread near edges of these dough hearts, leaving a 1⁄8-inch border and a 1⁄4-inch circle in centers.
Pipe marshmallow crème into center circles.
Using a paring knife, cut a 1⁄4-inch “X” in centers of remaining 20 dough hearts.
Place a cut dough heart on top of chocolate spread layer of each prepared dough heart.
Using a fork dipped in flour, crimp together edges of dough hearts to seal. Lightly brush tops with egg wash.
Place 1 inch apart on prepared baking sheets.
Bake until golden brown, 15 to 18 minutes.
Let cool for 5 minutes. Transfer to wire racks and let cool completely.
Place Royal Icing in a piping bag, and cut a 1⁄8-inch hole in tip of piping bag.
Pipe icing on top of tarts as desired.
Serve immediately, or place in a single layer in an airtight container and serve within a day.