In a food processor, pulse half of the hazelnuts until finely chopped. Add 2 cups of the flour, 3 tablespoons of the sugar and the salt and process until the nuts are finely ground. Add all but 2 tablespoons of the butter and pulse until the mixture resembles coarse meal. Whisk the egg yolk into the ice water, add to the processor and pulse until the pastry comes together. Turn the pastry out onto a lightly floured surface and pat it into a 1-inch-thick disk. Wrap in plastic and refrigerate for at least 1 hour.