Place hazelnuts and dairy in a blender. Blend until nuts are coarsely chopped up and the dairy is all frothy. Refrigerate dairy overnight. Strain through a fine strainer, pushing down on solids. You should end up with 4 cups of dairy.
Whisk eggs and yolks together with salt and half the sugar.
Heat the 4 cups of dairy with the rest of the sugar until just below a boil. Temper all the dairy into the egg mixture, and then strain into a large pitcher. Do not continue to cook.
Whisk in the vanilla and liqueur. Taste for salt, and add more, if necessary.
Cook some sugar to a medium amber color, then pour about 1 1/2 TBSP into individual ramekins or pour in enough to just coat the bottom of a larger container (maybe a 9" cake pan).
Place ramekins or pan into a larger pan lined with a kitchen towel. Fill ramekins to within 1/4" of the top (fill cake pan w/all the mixture) and put in the larger pan. Place the pan on the oven rack and carefully pour hot water into the pan to come halfway up the sides of the ramekins.
Cover the pan with a baking sheet or with a large piece of heavy duty foil. Don't let the foil touch the custard.
Bake at 275 degrees until just set. This could take awhile--maybe up to an hour for individual ones and even longer for a big old flan. A knife inserted into the center should come out clean, but the custard should still have just a bit of wiggle in the center.
Carefully remove from oven and cool at room temperature for an hour or so. Refrigerate until cold.
To serve, run a thin knife around the outside of the ramekins/pan, firmly hold a plate on top and flip the whole thing over. Lovely caramel should pool around the flan. Serve with a wee quenelle of whipped creme fraiche and some sort of a crunchy thing--hazelnut streusel would be nice. Or maybe biscotti w/hazelnuts.
Notes
If you don't need 12 4 oz servings, you could make 8 6 oz servings or you could just half the recipe to make 6 4 oz servings. You can also bake as one large flan and serve it sliced. Baking time will increase significantly.