1
Preheat the oven to 180°C/160°C fan/350°F/Gas 4 and line one large or two smaller baking trays with baking parchment.
2
Put the butter into a pan and melt over a low heat, then allow to cook until it smells lovely and nutty. Pour into a shallow dish or bowl and chill until set but not hard ( the consistency of softened butter).
3
Beat the butter with the sugars in a free-standing mixer on a high speed until light and fluffy. This will take about 5 minutes.
4
Stir the flour, salt and bicarbonate of soda together in a medium bowl. Add a little of this flour mix to the butter mixture, add the egg and mix to incorporate the egg. Reduce the speed to low and add the rest of the flour mixture along with the hazelnuts and chocolate.
5
Spoon dollops of the mixture (a small ice-cream scoop works brilliantly here) onto the lined trays, leaving a good amount of space between them as they will spread when they bake. Bake for 12-14 minutes until even and golden. You may need to rotate or turn the trays halfway, depending on your oven.
6
Remove from the oven and sprinkle some flaky sea salt over the cookies. Leave to cool on the trays for at least 5 minutes before devouring.