Thaw the puff pastry for about 45 minutes on counter until soft enough to roll out, but still very cold.
Meanwhile, cut a heart template from paper that is about 11 inches tall and wide.
Preheat oven to 400 degrees.
Unfold one piece of pastry onto a piece of parchment paper and roll it out slightly until it's just bigger than the heart. You can cut off one corner of the pastry and place it along the bottom to accommodate the bottom of the heart.
Use the tip of a sharp knife (or a pizza cutter) to cut out the heart, and repeat with the other pastry.
Place one heart on a cookie sheet (still on parchment paper). Spread it with 1/2 cup Nutella. Top with the other heart. Optional: use a small heart cookie cutter to make a heart shape in the center of the heart.
Make cuts from the edge of the heart almost to the middle, about 1 inch apart (or to the heart shape you made with the cookie cutter), all the way around the heart. Leave the pieces attached in the middle.
Twist each piece one full rotation as shown in the blog post.
Beat one egg with 1 tablespoon water and brush over surface of heart.
Bake 20 minutes until puffed and golden brown.
Sprinkle with powdered sugar and drizzle with jam if desired. Serve warm or at room temperature.
Pastry tastes best if eaten within a few hours.