1 3/4 cup all-purpose flour, plus more for dusting
1/4 teaspoon kosher salt
1/2 cup unsalted butter, softened
3/4 cup sugar
3/4 teaspoon vanilla
1 large egg
Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In a food processor, grind the toasted hazelnuts until finely ground. Add the flour and salt and pulse to combine.
Using an electric mixer, cream the butter and sugar until it's pale and fluffy. Add the vanilla and egg and beat well to combine. With the mixer on low, gradually add the dry ingredients to the wet ingredients, beating until combined.
Turn the cookie dough out onto a lightly floured surface. Using a lightly floured rolling pin, roll the dough out until it's 1/4-inch thick. Use a cookie cutter to cut out tiny hearts. Place the hearts on the baking sheets (they can be pretty close together), and bake until they're lightly browned on the bottom, about 10 minutes.
Once all of the cookies are baked, reduce the oven to 250°F and bake for an additional 20 to 30 minutes to really crisp them up. Let cool on the pans.