In a medium saucepan, combine the milk, butter, sugar, and salt. Bring to a boil over medium heat, stirring with a heatproof spatula. Once the mixture reaches a boil and the butter is fully melted, remove from the heat and add the flour all at once. Stir vigorously with a wooden spoon until the dough comes together in a smooth ball that pulls away from the sides of the pan. Return the pan to medium heat and cook the dough, stirring constantly, for about 2 minutes to dry it out slightly. It should leave a thin film on the bottom of the pan.
Transfer the dough to the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed for a few minutes to cool the dough. When the bowl is no longer warm to the touch, begin adding the eggs one at a time, mixing well after each addition. After adding 3 eggs, whisk the fourth in a small bowl and add just enough to achieve a smooth, pipeable dough. The texture should stretch between your fingers without tearing.
Preheat the oven to 350°F (180°C), conventional heat. Line a baking sheet with parchment paper or a silicone mat.
Spoon the dough into a piping bag fitted with a large star tip. Pipe a 4-inch/10 cm circle, then pipe a second ring right next to it, touching the first. Pipe a third ring directly on top, centered over the seam of the two bottom rings to build height. Sprinkle the top generously with sliced almonds. Bake for 45 minutes, or until puffed and deeply golden. Do not open the oven during baking.
Remove from the oven and let cool completely on a wire rack.
Use a serrated knife to slice each Paris Brest horizontally through the center to create a top and bottom half.
To prepare the cream, place the softened butter in a stand mixer fitted with the whisk attachment. Beat on high speed for 2–3 minutes, until light and airy. Add the hazelnut paste and continue to whip until fully combined and fluffy. Stop the mixer, add the chilled pastry cream, and whip again briefly—just a few seconds—until the cream is thick, smooth, and stable. Transfer the cream to a clean piping bag fitted with a plain round tip.
To assemble, spread a thin layer of hazelnut paste over the bottom half of each pastry. Pipe the hazelnut cream generously on top. Close with the top ring, press gently, and chill for at least 4 hours to set.
Before serving, dust with powdered sugar and garnish with small dollops of hazelnut paste and a handful of candied hazelnuts. Slice and serve cold or slightly chilled. The finished Paris Brest can be stored in the refrigerator for up to 3 days.