Line 2 baking sheet pans with parchment paper, set aside. Pulse the hazelnuts in a food processor or a coffee grinder until mostly flour-like with some pieces that still retain texture.
In a large mixer bowl whisk together ground hazelnuts, flour, sugar, cornstarch, and salt. Add the cold cubed butter, vanilla extract and mix until a dough forms with no visible butter chunks.
Using a small ice cream scoop, portion the dough into about 2 teaspoon chunks, then roll each one into a ball. You'll get about 40 dough balls. Set Italian cookies onto the prepared baking sheet about 1.5 inches apart.
Slightly tap on top of each hazelnut cookie ball to flatten. See *recipe notes* for how to shape and bake smaller sized cookies.
Chill cookies in the fridge for 1 hour. About 45 minutes into it, preheat the oven to 325 degrees with the baking rack in the middle. Bake cookies for about 17 to 18 minutes, or until just starting to brown on top. Remove from oven and cool completely.