For the craquelin

1. In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar on medium speed until combined. Sift together flour, cocoa powder and salt and add to the bowl. Mix until incorporated and becomes a coarse meal. Place dough between two sheets of parchment paper. Roll to a 1-2 mm thickness. Place on a sheet pan and place in the freezer for at least 15 minutes.

For the Nutella pastry cream

1. Line a sheet pan with plastic wrap and set aside.
2. In a medium saucepan, combine 3/4 cup (170 g) of the milk, the 2 tablespoons (25 g) butter, 2 1/2 tablespoons (32 g) of the sugar, the vanilla extract.  Set over medium heat and bring to a boil.
3. Meanwhile, in a medium bowl, whisk together cornstarch, all-purpose flour, and remaining 2 1/2 tablespoons (32 g) granulated sugar.  Whisk in the remaining 1/4 cup (56 g) of milk.  Whisk in egg yolks until combined.
4. Once boiling, take the vanilla mixture off of the heat.  Pour half of the milk mixture into the egg mixture while whisking vigorously to prevent the egg yolks from cooking.  Pour the egg mixture through a sieve into the saucepan with the remaining vanilla mixture.
5. Return saucepan to medium heat and bring pastry cream to a boil while whisking constantly.  Allow to cook for 1 minute and then remove from heat.  Whisk in Nutella.
6. Transfer pastry cream to the plastic-lined sheet pan.  Spread mixture into an even, thin layer.  Place plastic wrap directly on top and refrigerate until cool to the touch.

For the dough

1. Preheat oven to 400°F/200°C. Use a 2-inch round cookie cutter to trace a total of 20 circles on two sheets of parchment paper allowing 2-inches between each circle. Place the traced sides down into 2 large sheet pans. Set aside.
2. Make pâte à choux dough.
3. Fill a large piping bag fitted with a large round piping tip with the dough.  Pipe mounds of pâte à choux onto traced circles.  If the dough has left a “tail”, flatten down with a wet fingertip.
4. Remove the craquelin from the freezer. Use the 2-inch round cutter to cut out circles of dough, enough for 1 sheet pan of puffs. Transfer circles to the tops of each pâte à choux piped dough and slightly push down. Place the remaining craquelin dough in the freezer.
5. Bake the cream puffs in the preheated oven for 15 minutes, then turn down the oven to 350°F/177°C, and bake for an additional 20 minutes, until the puffs are a deep golden.  Place on a cooling rack to cool completely.
6. Repeat steps 4 and 5 with the second sheet pan of dough.

For the chocolate whipped cream

1. In a large bowl using a hand-held mixer or stand mixer with the whisk attachment, whisk the heavy cream, cocoa powder, powdered sugar and vanilla on medium-low speed until mixture becomes frothy.  Increase speed to medium-high and continue whisking until medium peaks form, about 3 minutes.  Careful not to over mix or else it will become thick and lumpy.

To assemble

1. Transfer the pastry cream to a piping bag fitted with a round tip.
2. Using a serrated knife, cut the tops off the cream puffs, about one quarter away from the top.
3. Pipe a layer of pastry cream into the bottom of the cream puff, top with a piping of the chocolate whipped cream and place the pâte à choux lid on top.
4. Refrigerate until ready to serve.