For the crackle topping, beat together the butter, demerara and plain flour until smooth and homogenous. Roll out between two sheets.
Preheat the oven to 200°C.
Add the milk, water, salt, sugar and butter to a large saucepan and heat over medium to melt the butter, then bring to the boil for 1 minute.
Add the flour in one go and beat to combine - it should make one large lump and come away from the sides. Cook for at least 30 seconds on the heat to drive off some moisture, then add to the bowl of a stand mixer to cool until warm to the touch but not too hot.
Slowly add the egg in 4-5 additions, beating well between each, until the choux is thick, glossy and has a dropping consistency which should make a ‘V’ off the back of the beater.
Add to a piping bag with a large round nozzle.
Prepare a large baking tray with a silicone mat or non-stick baking paper.
Using a small cutter, cut out the crackle topping circles, set aside.
Use the same cutter dipped in a little plain flour to mark on your baking tray the size of the choux buns you are going to pipe. Pipe the choux pastry, then top with a crackle circle.
Bake for 15-20 minutes at 200°C until risen and golden. Then reduce the temperature to 150°C, pierce a small hole in each choux bun, then return to the oven for a further 10-15 minutes to dry out. Cool on a wire rack.
When cool, add the cream, sugar and vanilla to a large bowl and whip the cream to very soft peaks - as you are going to pipe it, you don’t want it to be stiff or it will be over-whipped when piped.
Cut the choux buns in half with a small serrated knife, fill with piped cream and sandwich together.
Drizzle with salted caramel sauce and sprinkle with roasted chopped hazelnuts.