• about Preheat oven to 425 degrees Fahrenheit. Get out 2 baking sheets and cut 2 sheets of parchment paper to match. Set aside.
• In a medium heavy-bottomed pot over medium heat, combine the milk and half the sugar. Heat until it just starts to bubble, then remove from heat.
• In a medium mixing bowl, beat the egg yolks, remaining half of the sugar, cornstarch, and vanilla extract.
• While whisking, slowly dribble about 1/3 of the hot milk into the egg mixture. Beat vigorously to temper the eggs.
• Whisk the egg mixture into the remaining hot milk and return to the stove.
• Heat over medium flame until it starts to bubble, then continue cooking for 3 minutes to cook off the starch taste from the cornstarch.
• Remove from heat and add the Nutella, stirring until well incorporated.
• Transfer to another container and cover the surface with plastic wrap. Refrigerate until cold, up to 24 hours in advance.
• Take out the puff pastry sheets. My oven is a small wall oven, so I can't even fit a half sheet pan in it. Hence, I had to bake 1 puff pastry sheet at a time.
• Unfold one puff pastry sheet.
• Lay out a piece of parchment paper on one of the baking sheets and place the puff pastry in the middle of it.
• Lay the other piece of parchment paper over the top and place the other baking sheet on top to keep it from puffing up too much in baking.
• Bake on the center rack for 18 minutes, then remove the top sheet and bake for an additional 3-4 minutes, or until lightly browned.
• Remove to a cooling rack to cool completely.
• Repeat with remaining sheets of puff pastry until you have 3 squares.

Assembly

• Once you are ready to assemble, prepare the glaze in a medium mixing bowl. You'll only want to add enough milk to get it to a thick, oozy consistency. You don't want it to be so thin that it drips right off. So be careful, add the milk 1 Tablespoon at a time, and stir thoroughly between each addition. I only needed about 2 Tablespoons for the consistency I got, but it could vary for you.
• Place the chocolate chips in a zip top bag and place that in a microwavable bowl.
• Microwave on medium heat for 30 second intervals, squeezing the contents in between each to help it melt more evenly.
• Lay out a piece of puff pastry and spread over it half the Nutella pastry cream.
• Place another sheet of puff pastry on top and repeat with the remaining pastry cream.
• Place the remaining sheet of puff pastry on the top and spread the glaze over that.
• Snip the corner off the zip top bag with the chocolate creating a very small hole. Pipe lines of chocolate about an inch or two apart.
• Using a toothpick, drag lines perpendicular to the chocolate every inch or so, alternating directions.
• Chill for at least 3 hours before serving. Trim the edges before cutting to serve. Make sure you're using a very sharp knife, as this quickly gets messy.