First, make the craquelin by creaming the butter and sugar together then adding the flour. Mix until it resembles breadcrumbs.
Using the heat from your hands, bring the dough together and roll to 3mm between two sheets of baking paper. Chill.
To make the choux, bring the milk, water, butter, sugar and salt to a boil.
Take off the heat and stir in the sifted flour.
Once the flour is incorporated fully, return to the heat and stir for about two to three minutes on a medium heat.
Transfer the mixture into a standing mixer and turn up the speed to medium high. Add one egg at a time making sure that each egg is fully incorporated before adding the next.
Transfer the mixture into a piping bag fitted with a star nozzle.
Pipe your desired shape and cut out a piece of craquelin to fit neatly on top.
Bake for 20 minutes at 180 degrees Celsius.
Whip up the chestnut mousse and assemble the Paris Brest.
