Make the praline:
1 cup hazelnuts
1 cup sliced almonds
½ cup sugar
Preheat the oven to 350 degrees.
Place the hazelnuts and almonds in a shallow baking pan and toast them in the oven until golden, 8 to 10 minutes. Let cool.
In a small, heavy skillet set over medium heat, cook the sugar, swirling the pan, until it begins to melt, about 2 minutes. Continue to cook, swirling the skillet, until the sugar is melted to a deep golden caramel, 2 to 3 minutes more. Remove the pan from the heat and, working quickly, stir in the nuts until they are coated, then transfer mixture to a greased baking pan, spreading it slightly. Let it stand at room temperature until hardened and cool, about 30 minutes. Transfer the praline to a sealable plastic bag, press out excess air, and seal. Coarsely crush the praline in the bag using a rolling pin or the bottom of a heavy skillet, then transfer it to a food processor and puree until it becomes smooth and creamy.
Make the cream filling:
1 cup whole milk
3 large egg yolks
⅓ cup sugar
3 tablespoons cornstarch
¼ teaspoon salt
2 tablespoons unsalted butter
1 teaspoon vanilla
¾ cup chilled heavy cream
In a 3-quart saucepan set over medium heat, bring the milk to a simmer. While the milk heats, whisk together the yolks, sugar, cornstarch, and salt in a heat-proof bowl. Add the hot milk to the yolk mixture in a stream, whisking, then transfer the mixture to the saucepan and bring to a simmer over medium heat, whisking (the mixture will become thick and lumpy). Simmer, whisking constantly, for 3 minutes (the mixture will become smooth). Remove from the heat and stir in the butter and vanilla. Transfer the mixture to a bowl and press plastic wrap on the surface to prevent a skin from forming. Refrigerate.
Beat the heavy cream in an electric mixer until it holds stiff peaks. In a large bowl, beat the pastry cream with a mixer until smooth, then add the praline mixture and beat until incorporated. Gently fold in the whipped cream, ⅓ at a time, until it is thoroughly mixed. Cover the surface of the praline cream with plastic wrap and chill until ready to use.
Make the choux pastry:
1 cup water
1 stick unsalted butter, cut into ½-inch cubes
1 teaspoon sugar
½ teaspoon salt
1 cup flour
4 large eggs
1 large egg yolk, lightly beaten with 1 tablespoon water
3 tablespoons sliced almonds
1 tablespoon confectioners sugar, plus more for dusting
Special equipment: Pastry bag with a ⅝-inch plain tip and a ½-inch open-star tip.
Preheat the oven to 375 degrees.
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 the paper over (circles will still be visible) and place on a large baking sheet.
In a 3-quart saucepan, bring the water, butter, sugar, and salt to a boil over high heat. Reduce the heat to medium. Add the flour all at once and cook, stirring vigorously with a wooden spoon, until the mixture pulls away from the side of the pan, about 1 minute. Continue to cook and stir vigorously (to dry out the mixure) about 3 minutes more. Remove the pan from the heat and cool the mixture, stirring occasionally, until it is warm to the touch, 5 to 10 minutes. Add the whole eggs 1 at a time, stirring vigorously after each addition until the dough is smooth.
Transfer the dough to a pastry bag fitted with a plain tip and pipe 3 concentric rings to fill space between the traced circles on parchment, then pipe 2 more on top to cover seams between bottom rings. Lightly brush the pastry with the egg wash, then scatter the almonds over the pastry and dust with powdered sugar.
Bake the choux pastry until it is golden and well-puffed, 20 to 25 minutes, then reduce the oven temperature to 350 degrees. and continue to bake until deep golden and firm to the touch, about 20 minutes. Transfer the pastry on parchment to a rack and cool completely.
Halve the pastry horizontally with a serrated knife and carefully invert the top onto a work surface. Transfer the praline cream to the cleaned and dried pastry bag fitted with the star tip, and pipe cream decoratively into the bottom half of the pastry. Carefully place the top piece over it. Dust with more powdered sugar.