Start with the craquelin. Beat together the butter and brown sugar then mix in the salt and flour. Place into the fridge for 15 minutes then roll out thin on a floured surface. Cut into discs the same size you want to make the eclairs and buns. Place onto a baking tray and then back into the freezer. Meanwhile, make the choux by beating together the butter and sugar then stir in the salt and flour and mix to a dough. Chill for 15 minutes then roll out thin and cut into discs and oblong. Heat the water, milk and butter in a saucepan and melt together. Stir in the salt and flour and mix together to form a roux and cook for a minute. Allow to cool down slightly then place into a mixer, add in the eggs gradually and mix to a soft paste. Pipe into 2 rings on non-stick paper, oven at 190°C for approximately 30 minutes (Do not open the door before 30 minutes) Remove from the oven and allow to cool down completely and make a small hole in the bottom to fill. Whip the double cream then whisk in the biscoff spread. Spoon the cream filling into a piping bag and use to fill the choux. Pipe more cream decoratively onto the top and decorate with toasted hazelnuts. Dust with icing sugar and chill before serving.