For the pastry:

Preheat oven to 375°F. Line the baking sheet with parchment paper.

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 and using the fork prick inside the puff 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 milk 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.

For the filling:

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.

Pour the filling into the cooled pastry crust, and smooth the top. Transfer to the fridge for at least an hour for the filling to chill and firm up.

Assemble the tart:

Once chilled, top the tart with the sliced strawberries. Sprinkle with chopped hazelnuts, if desired. Refrigerate or serve immediately.