For the pastry cream, heat the evaporated milk and milk in a medium saucepot until just below a simmer. In a bowl, whisk the egg yolks, and cornstarch together. Place the chocolate and butter in a separate bowl and place a strainer over the top.
Pour the hot milk over the egg mixture while whisking constantly until it has all been added. Return this to the pot and whisk over medium heat until the custard thickens and turns glossy, about 3 minutes. Remove the pot from the heat and pour it through the strainer over the chocolate and butter. Whisk this until the chocolate has melted and the custard is smooth. Whisk in the hazelnut liqueur and vanilla, then whisk in the hazelnut butter. Place a piece of plastic wrap directly on the surface of the custard, cool to room temperature, then chill until set, about 3 hours.
Preheat the oven to 400F and line a baking tray with parchment paper, and have a second baking tray of the same size on hand.
Cut the pastry dough into 3 equal pieces. On a lightly floured surface roll out each piece to a rectangle about 5-x-14 inches. Alternatively, you can roll out each piece to about 4-x-8 inches and then finish rolling the dough through a pasta roller set to the widest setting (this makes for a pastry that bakes evenly and level). If needed, stack the 3 pieces of dough and trim the edges so that they are all the same size (they will be closer to 4-x-12 inches once trimmed). Chill the dough for 15 minutes.
Place the 3 pieces of dough on the prepared baking tray and dock with a fork. Place a second sheet of parchment paper over the pastry and place the second baking tray on top. Bake the pastry for 10 minutes, then add a heavy weight on top of the top baking tray (a cast iron skillet, or even 2 bricks wrapped in foil) and bake 10 minutes more. Remove the weights, the top baking tray and parchment and finish baking for about 10 more minutes, until the pastry is a rich golden brown. Cool completely before assembling.
To assemble, place one of the pastry pieces on a platter or flat board. Stir the pastry cream to soften and spread half of it over the pastry. Top this with a second sheet of pastry and spread the remaining half of the pastry cream overtop. Top this with the third piece of pastry and press it down ever so gently. Chill for an hour.
Dust the top of the napoleon generously with icing sugar. Using a sharp Chef’s knife, trim the edges of the pastry if needed, then cut the napoleon into 6 individual portions, each 4-x-1 ½-inches. Store the napoleons refrigerated until ready to serve.
The napoleons can be prepared up to a day in advance, and stored refrigerated up to 2 days.
You can use ready made pastry or make it yourself using Anna Olson's recipe for puff pastry.