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For the pastry cream
Make a pastry cream following the directions on this page.
For the hazelnut paste
Preheat the oven to 330°F (165°C). As with the praline, toast the
hazelnuts for 15 to 20 minutes, then blend them in a food processor,
adding the confectioners’ sugar and fleur de sel.
For the French buttercream
Using the milk, egg yolks, and superfine sugar (1), make a crème
anglaise following the directions on this page. In the bowl of an
electric stand mixer fitted with a whisk, while whipping, gradually add
to the butter to the pastry cream.
Whip the egg whites.
In a saucepan, mix ⅓ cup (78 g) of water and the superfine sugar
(2). When the temperature reaches 250°F (120°C), pour it over the
whipped egg whites. Blend in an electric stand mixer at medium
speed until it cools. Then use a silicone spatula to mix the two
mixtures together.
For the Paris-Brest cream
Smooth the pastry cream and then add the hazelnut paste. Whisk
the buttercream until smooth. Gently add it to the first mixture. Set
aside in a gastronorm pan.
For assembly and finishing
Preheat the oven to 360°F (180°C). Use a ⅜-inch (8-mm)-tip to pipe
2¾-inch (15-cm)-long lines of the choux paste onto a baking sheet
lined with parchment paper. Add the hazelnuts on top and bake in
the oven for 30 minutes. Cool on a rack. Fill them with hazelnut
praline from the bottom.
Using a pastry bag fitted with a ½-inch (12-mm) tip, pipe 2¾-inch
(15-cm)-long lines of Paris-Brest cream, and freeze for 1 hour.
Place a line of Paris-Brest cream on a piece of choux pastry, then
add a second piece of choux pastry. Defrost for 30 minutes.