Firstly make the praline by placing the caster sugar in a small heavy based saucepan and placing over a gentle heat until the sugar dissolves and starts to caramelise. It will turn to a golden brown. Remove from the pan from the heat, stir in the hazelnuts and pour onto a baking tray lined with oiled baking paper.
Leave the praline to cool completely, transfer to a food processor and blitz until powdered and just starting to form soft clumps. Store in an airtight container.
Now make the creme patissiere as this needs to chill.
Place the milk and the vanilla pod in a saucepan. Bring slowly to the boil.
Put the egg and egg yolk into a mixing bowl with the sugar and flour. Whisk together until smooth.
Remove the vanilla pod from the hot milk (you can wash it under the cold tap and dry it to use again) and pour the milk on to the egg mixture, whisking as you pour.
Return this custard back to the saucepan and place over a gentle heat.
Bring to the boil, whisking well as it thickens to prevent lumps forming.
Remove from the heat and sprinkle with a little caster sugar to prevent a skin forming.
Cover with cling film and chill in the fridge until required.
Now make the choux pastry rings -
Preheat the oven to 220C/Gas Mark 7.
Put the water and butter in a saucepan over a low heat until the butter has melted then bring to a rolling boil.
As soon as boiling point is reached add the flour in one go and beat very well with a wooden spoon until the mixture forms a smooth ball and leaves the sides of the pan.
Leave to cool for a few minutes. (standing the pan in a bowl of cold water for a minute will help)
Beat the eggs lightly and add them to the cooled dough a little at a time, beating well as you go. (You can use an electric mixer if you prefer.)
The pastry should be stiff enough to place in a piping bag without running out, so stop adding egg if this stage is reached before the egg is used up. The choux pastry should look slightly glossy.
Place the choux pastry into a piping bag with a large nozzle fitted and pipe 8cm circles onto a baking sheet lined with baking paper. You can draw out the circles onto the paper if it helps.
Place in the pre-heated oven and bake for 10 minutes before turning the oven down to 190C/Gas Mark 5.
Bake for a further 20-25 minutes until the pastry is brown and crisp and will have risen to double the size.
Remove from the oven and make a little slit in the side of each pastry ring to allow the steam to escape.
Once the baking is finished I usually return all the choux rings to the cooling oven to dry out for a further 10 minutes.
Now finish the Praline mousseline cream filling while the pastry cools.
Place the unsalted butter into a mixing bowl and beat well until soft and creamy.
Slowly add the chilled creme patissiere a spoonful at a time to prevent lumps forming until all the custard has been whisked in.
Add the 70g of the powdered praline to the custard and mix well.
Now assemble the Paris Brest by piping the mousseline into the pastry rings. The easiest way is to split the pastry rings in half horizontally, filling each half and sticking them back together.
Dust with icing sugar before serving.