Whisk together the butter, brown sugar and egg until paler in colour. You can also use an electric mixer.
Add the vanilla essence, flour and salt and incorporate using a spatula.
Fold in the white chocolate (chopped if needed) and hazelnuts, leaving a few hazelnuts for the top.
Line a 20x20cm mould with baking paper and pour the mix in. Level it using a spatula and sprinkle the hazelnuts on top. If you want to add white chocolate to the top as well do so when there's only 1 minute left on your timer or else the chocolate will caramelize and go hard.
Take to a preheated oven at 180C for 15min.