PreparationMake praline: Put oven rack in middle position and preheat oven to 350°F. Toast hazelnuts in a shallow baking pan in oven until skins split and nuts are golden, 10 to 15 minutes. Remove from oven (leave oven on), then wrap hazelnuts in a kitchen towel and let steam 5 to 10 minutes. Rub hazelnuts in towel to remove loose skins (some skins may not come off), then transfer hazelnuts to a small bowl, discarding skins. While hazelnuts steam, toast almonds in baking pan until golden, 5 to 8 minutes, then add to bowl with hazelnuts. Lightly grease pan and set aside. Cook sugar in a dry small heavy skillet over moderate heat, swirling skillet, until sugar begins to melt, about 2 minutes. Continue to cook, swirling skillet, until sugar is melted into a deep golden caramel, 2 to 3 minutes more. Remove from heat and, working quickly, stir in nuts to coat, then transfer mixture to greased baking pan, spreading slightly. Let stand at room temperature until hardened and cool, about 30 minutes. Transfer praline to a heavy-duty sealable plastic bag and seal bag, pressing out excess air. Coarsely crush praline in bag using a rolling pin or bottom of a heavy skillet, then transfer three fourths to a food processor and purée until it becomes a smooth, creamy "butter," 3 to 4 minutes. Reserve remaining crushed praline for garnish. Make cream filling: Bring milk to a simmer in a 2 1/2- to 3-quart heavy saucepan over moderate heat. While milk heats, whisk together yolks, sugar, cornstarch, and salt in a heatproof bowl. Add hot milk to yolk mixture in a stream, whisking, then transfer mixture to saucepan and bring to a simmer over moderate heat, whisking (mixture will become thick and lumpy). Simmer, whisking constantly, 3 minutes (mixture will become smooth). Remove from heat and stir in butter and vanilla. Transfer to a clean bowl and chill pastry cream, its surface covered with wax paper, until cold, at least 1 hour. Beat heavy cream in a bowl with an electric mixer until it just holds stiff peaks. Beat pastry cream in a large bowl with mixer until smooth, then add praline "butter" and beat until incorporated. Fold in whipped cream, one third at a time, gently but thoroughly, then cover surface of hazelnut cream with wax paper and chill until ready to use. Make choux pastry: Put oven rack in middle position and preheat oven to 425°F. Trace a 9-inch circle with a pencil on a 12-inch square of parchment or wax paper, then trace a 5-inch circle inside it. Turn paper over (circles will still be visible) and put on a large baking sheet. Bring water to a boil with butter, granulated sugar, and salt in a 3-quart heavy saucepan over high heat, then reduce heat to moderate. Add flour all at once and cook, stirring vigorously with a wooden spoon, until mixture pulls away from side of pan, about 1 minute. Continue to cook and stir vigorously (to dry out mixture) 3 minutes more. Remove pan from heat and cool mixture, stirring occasionally, until warm to the touch, 5 to 10 minutes. Add whole eggs 1 at a time, stirring vigorously after each addition until dough is smooth. Transfer dough to pastry bag fitted with plain tip and pipe 3 concentric rings to fill space between traced circles on parchment, then pipe 2 more on top to cover seams between bottom rings. Lightly brush pastry with some egg wash, then scatter almonds over pastry and dust with 1 tablespoon confectioners sugar. Bake choux pastry until golden and well puffed, 20 to 25 minutes, then reduce oven temperature to 375°F and continue to bake until deep golden and firm to the touch, about 25 minutes more. Immediately prick top of pastry in 8 to 10 places with tip of a small sharp knife (to release steam) and continue to bake until golden brown, about 10 minutes more. Transfer pastry (on parchment) to a rack and cool completely, about 30 minutes. Halve pastry horizontally with a serrated knife and carefully invert top onto work surface. Remove and discard any wet dough from interior of top and bottom. Transfer hazelnut cream to cleaned and dried pastry bag fitted with star tip and pipe cream decoratively into bottom half of pastry, then carefully reinvert top half over it. Sprinkle top with reserved praline and dust with additional confectioners sugar. Cooks' notes:Praline and praline "butter" can be made 2 days ahead. Chill butter in an airtight container and keep remaining praline in an airtight container at room temperature.Pastry cream (without praline "butter" or whipped cream) can be chilled, its surface covered with wax paper and bowl covered with plastic wrap, up to 2 days.Choux ring can be baked (but not halved) 8 hours ahead and cooled completely, uncovered, then kept, loosely covered with foil (not plastic wrap), at room temperature. Cooled choux ring can also be frozen, tightly wrapped in plastic wrap, 1 week. Thaw completely in wrapping before recrisping in a preheated 350°F oven 10 minutes. Cool to room temperature before proceeding.Cream filling can be chilled up to 4 hours.Paris-Brest can be assembled 2 hours before serving and kept at cool room temperature.