Heat the oven to 180 C / 350 F. Spread the 100 g hazelnuts on a 33 x 46 cm / 13 x 18-inch sheet pan and toast for 10 to 12 minutes, until the skins split and the nuts smell fragrant. Tip them into a clean towel, rub off most of the skins, and let them cool. Raise the oven to 200 C / 390 F and line that pan and a second 33 x 46 cm / 13 x 18-inch pan with parchment.

Combine the water, milk, butter, 1 tsp sugar, and salt in a 2-quart / 1.9 L saucepan. Bring to a full boil over medium heat so the butter melts completely, then tip in the flour all at once. Stir hard with a wooden spoon for 2 minutes, until the dough pulls from the sides and leaves a thin film on the pan bottom. Transfer to a mixing bowl and cool for 5 minutes.

Beat the 4 eggs together, then add them to the dough in 4 additions, beating until each is fully absorbed before adding the next. Stop when the dough is smooth and drops from the spatula in a slow V shape. You may have a spoonful of egg left over.

Spoon the dough into a piping bag with a 1 cm / 3/8-inch round tip. Pipe 24 mounds, 12 per pan, each about 3 cm / 1.25 inches wide and 4 cm / 1.5 inches apart. Smooth any peaks with a wet fingertip.

Bake with the pans on two racks for 10 minutes at 200 C / 390 F. Reduce to 180 C / 350 F and bake 10 minutes more, then swap the pans and bake 5 minutes, until the buns are deep golden and feel light and hollow. Keep the door shut for the first 20 minutes. Pale buns collapse as they cool, so add 3 to 5 minutes if the sides still look blond. Pierce each bun with a skewer, turn the oven off, and leave the buns inside with the door ajar for 5 minutes. Move them to racks and cool completely, about 30 minutes.

While the buns bake, stir 100 g sugar and 35 ml water in a 20 cm / 8-inch heavy-bottomed saucepan over medium heat until the sugar dissolves. Stop stirring and boil for 8 to 10 minutes, until deep amber, about 170 C / 338 F. Off the heat, stir in the toasted hazelnuts and pour onto a sheet of parchment set on a heatproof board. Let it harden for about 20 minutes.

Break the cooled brittle into pieces. Crush about 80 g with a rolling pin into small chunks for topping. Process the remaining 120 g in a food processor for 8 to 10 minutes, scraping the bowl every couple of minutes, until the crumbs turn into a thick, loose paste.

Heat the 100 ml cream in the rinsed 2-quart / 1.9 L saucepan until steaming. Pour it over the chopped chocolate and salt, wait 2 minutes, then whisk until glossy and smooth. Keep the sauce warm.

Whip the 300 ml cold cream and mascarpone with a hand mixer in a mixing bowl for 2 to 3 minutes, until soft peaks form. Stir a spoonful into the 120 g praline paste to loosen it, then fold the paste back into the cream. Spoon into a piping bag with a 6 mm / 1/4-inch round tip.

Poke a small hole in the base of each cooled bun with the piping tip and fill until it feels slightly heavy, about 1 tbsp of cream each. Set 3 buns on each plate, pour warm chocolate sauce over, and scatter the crushed praline on top. Serve within 30 minutes of filling.