Paris Brest: Preheat your oven to 350 degrees F (180 degrees C) and place the rack in the center of the oven. Line a baking sheet with parchment paper. Draw 7 - 3 1/2 inch (9 cm) circles on the parchment paper to use as a guide when piping the choux pastry.

Choux Pastry: Put the butter, milk, water, salt, and sugar in a medium saucepan, placed over medium heat, and bring to a boil. Remove from heat and, with a wooden spoon or heatproof spatula, stir in the flour. Place your saucepan back over medium-low heat and stir constantly until the dough comes away from the sides of the pan and forms a thick, shiny and smooth ball (about 1 minute).

Transfer the dough to your electric stand mixer, fitted with the paddle attachment (or you can use a hand mixer), and beat on low speed a minute or two to release the steam from the dough. Once the dough is lukewarm, add about 3/4 of the lightly beaten eggs (dough will separate and then come together) and continue to mix, on low speed, until you have a smooth thick paste (dough will fall from a spoon in a non-ragged "V" shape). (See video for demonstration.) If necessary, add more beaten egg until you get the correct consistency.

Transfer the dough to a large piping bag fitted with a 1/2 inch (1.25 cm) star tip (I used an Ateco #825). Pipe a 1 inch (2.5 cm) thick ring of dough just inside each 3 1/2 inch (9 cm) circle that is drawn on the parchment paper.

Egg Wash: In a small bowl whisk the egg. With a pastry brush, brush the glaze gently on the top and sides of the choux. Sprinkle with the shaved or sliced almonds. 

Bake for 20 minutes and then reduce the oven temperature to 325 degrees F (165 degrees C). Bake for a further 25-30 minutes or until the choux pastry is a golden brown color, puffed, and crisp. (Do not open the oven door while the choux pastry bakes or it might deflate.) Remove from oven and place on a wire rack to cool.

Pastry Cream: In a medium-sized heatproof bowl, whisk 1/2 the sugar (35 grams) with the cornstarch. Whisk in the egg yolks until you have a smooth paste.

Meanwhile, in a saucepan, bring the milk and remaining sugar (30 grams) just to boiling (just until milk starts to foam up.) Remove from heat and add slowly to egg mixture, whisking constantly to prevent curdling. (If you get a few pieces of egg (curdling) in the mixture, pour through a strainer.) Pour the egg mixture into a medium saucepan and cook over medium heat until boiling, whisking constantly. When it boils, reduce heat slightly and continue to whisk mixture constantly for another two minutes until it becomes thick. Remove from heat and immediately whisk in the vanilla extract (if using). Pour into a clean bowl and immediately cover the surface with plastic wrap to prevent a crust from forming. Cool to room temperature. If not using right away refrigerate until needed, up to 3 days. 

Hazelnut Praline Cream: Have the pastry cream and butter at the same temperature (between 65 - 70 degrees F (18 - 21 degrees C)). Place the pastry cream and butter in the bowl of your electric stand mixer, fitted with the paddle attachment (can also use a hand mixer). Beat on medium high speed until light and fluffy and very smooth (about 2 - 3 minutes). Stir the praline paste until smooth (as it is quite thick) and then add to the pastry cream mixture and beat until smooth. Place the Praline Cream in a pastry bag fitted with a large star tip (I used an Ateco #825).

To Assemble: Split the rings of choux in half horizontally using a serrated knife. Pipe the Praline Cream into the  bottom half of choux. If the Praline Cream is soft place the bottom halves of choux, filled with the Praline Cream, into the refrigerator until firm (about 20 - 30 minutes). Then place the top half of the choux on the Praline Cream and dust the top of each Paris Brest with powdered (confectioners or icing) sugar. Serve immediately or cover and refrigerate. Best served within 24 hours.

Makes 7 - 3 1/2 inch (9 cm) Paris Brest.