Preheat oven to 375°F.

Cut a piece of parchment paper the size of a standard baking sheet or use a silicone baking mat. Place the parchment paper or mat on a flat surface and very lightly dust it with flour. On the floured surface, roll out the puff pastry into a 9x11 inch rectangle.

Carefully lift the parchment paper/mat, with the pastry on it, onto a baking sheet.

Using the tip of a sharp knife, score a one-inch border around the pastry, being careful not to cut all the way through. This outer edge will puff up and hold in your filling and toppings once the tart is assembled. Brush the border with beaten egg and sprinkle with granulated sugar. Bake for 13-15 minutes, until golden brown and puffed.

Remove the puff pastry from the oven and rescore the border of the puff pastry, being careful not to cut all the way through. Gently press down on the center of the pastry to flatten it slightly. Allow the border to remain thick and puffed up. Cool to room temperature.

In a small bowl, whisk cornstarch, cocoa, and a half cup of milk. In a small/medium saucepan over medium heat, combine remaining milk (1 cup), Nutella, and salt, and whisk until the Nutella has melted. Add in the cornstarch and cocoa mixture, whisking while you pour. Bring the mixture to a boil, alternating between whisking and stirring with a rubber spatula. Once the mixture comes to a boil, boil for 2 full minutes, the mixture will thicken. Remove from the heat and whisk in 1 tablespoon strawberry jam.

Transfer your pastry crust to a cutting board or serving platter. It's easier to move now rather than after it's filled. Pour the filling into the cooled pastry crust (see note), and smooth the top. Transfer to the fridge for at least an hour for the filling to chill and firm up.

Once chilled, top the tart with the sliced strawberries. Microwave the strawberry jam for about 15 seconds, until it has thinned out. Brush the jam onto the top of the strawberries. Sprinkle with chopped hazelnuts, if desired. Refrigerate or serve immediately. I like to serve slices of the tart with a dollop of whipped cream.