For the history, the Paris Brest cake was created in 1910 by pastry chef Louis Durand, upon request from the organiser of a bicycle race between Paris and Brest. The cake has a large ring shape resembling bicycle wheels. This race began in 1891 and it's the oldest cycling event still being run.
Toast nuts in a 350ºF/180ºC oven for about 10 minutes. Let cool. Meanwhile, cook sugar to caramel stage, stir in nuts and cool onto silicone mat. Break into pieces (save some caramelized nuts for stuffing. Turn caramelized nuts into paste; set aside.
Bring water, milk, butter, sugar and salt to a boil in a medium saucepan. Remove from heat. Stir in flour and when homogenized, bring back to the stove and cook for a couple of minutes. Transfer to the bowl of an electric mixer fitted with the paddle attachment or whisk if using a Magic Mill Mixer. Mix on low speed for about 2 minute to get the steam out. Throw in eggs and mix until smooth.
On a parchment paper, draw a 6''/15cm circle; flip over parchment. Pipe a thin 1/2''https://bruno.b-cdn.net/1.2cm thick ring of dough just outside the 6'' circle. Pipe another ring outside the first ring making sure the ring of dough are touching. Pipe a final ring of pâte à choux on the top, along the center seam between the first two rings of dough. Dust the dough with powdered sugar and coat with sliced almonds. For the individual Paris-Brest, pipe out 16 ea. 3''/8cm diameter wheels on 2 large baking sheets (8 per tray) lined with a silicon mat or parchment using a 1/2''/1.2cm open star pastry tip. Unbaked piped pate a choux can be refrigerated a couple of days or frozen for weeks before being baked; place frozen wheels on a baking tray and bake right.
Bake at 350ºF/180ºC for 40 minute. Bake one sheet at the time. Do not open the oven door during baking or it will deflate. After baking time, turn oven off and leave the door ajar for 15 minutes to dry out.
Bring milk to a simmer. Meanwhile in a separate bowl, beat egg yolks with sugar to blend and add flour and corn starch. Turn off the heat. Whisk hot milk gradually into the yolk mixture. Return custard to the stove, bring to boil and cook for 2 minutes on medium high heat whisking constantly. Turn off the heat and whisk in the first the 60g butter. Transfer custard onto a tray lined with plastic wrap, cover with plastic wrap in contact and let cool and chill. Beat on high speed to smooth out. Then, mix in the praliné paste. Add 150g remaining butter and continue beating until creamy and fluffy; about 5 minutes. The praliné mousseline can be kept refrigerated for up to 4 days.
Split the pate a choux wheel in half horizontally. Pipe the praliné mousseline into the hollow center of the shell. Add some crushed caramelized nuts and pipe out more praliné mousseline. Chill. Dust some powdered sugar prior serving. Enjoy!