Position two racks in the upper and lower thirds of the oven. Pre heat the oven to 350 F. Line two baking sheets with silicone liners or parchment paper.

If necessary, use a food processor to grind the hazelnuts finely and measure out 1/2 cup if you don't have a scale (I just weighed the hazelnut meal). Heat the butter, sugar, instant espresso and corn syrup in a medium saucepan over low heat, stirring often, until the butter melts and the sugar dissolves. Increase the heat to medium high and, stirring constantly, bring the mixture just to a boil. Remove the pot from the heat and stir in the flour and salt until incorporated. Stir in the hazelnut meal and the vanilla. The mixture will thicken yet will still be quite loose while it is hot. As it cools, it will become firmer and firmer--and stickier, until finally it is easier to use your fingers to pinch the dough. Both consistencies are totally fine.

Drop the batter by the teaspoon 3 inches apart on the baking sheets, six cookies per baking sheet. Bake the cookies until evenly light brown, about 10 minutes. Oddly, despite starting out quite different colors, the espresso and plain versions look remarkably the same after being baked. About 5 minutes into baking, switch the sheets from top to bottom and back to front to promote even baking. The cookies won't begin to spread until about 6 minutes into baking.

Remove the cookies from the oven and let them set on the pan for 3-5 minutes. When they are firm enough to remove, use a wide spatula to transfer them to a cooling rack to cool completely. Wipe the pan with a paper towel before re-using.