Preheat oven to 350°. Toast hazelnuts on a rimmed baking sheet, tossing once, until golden brown, 1012 minutes. Remove nuts and reduce oven temperature to 200°. Bundle nuts in a kitchen towel and rub vigorously to remove skins. Spread out and let cool. Blend hazelnuts and salt in a food processor until a smooth, creamy nut butter forms (it should be pretty fluid; keep processing if still stiff); set aside. Using an electric mixer on high speed, beat egg whites and cream of tartar until frothy, about 1 minute. With motor running, gradually add granulated sugar and beat until medium peaks form, about 5 minutes. Gradually add powdered sugar and continue to beat until stiff, glossy peaks form, 810 minutes. Transfer meringue to a large bowl and gently fold in half of reserved hazelnut butter, leaving plenty of streaks. Add remaining hazelnut butter and fold once just to barely blend. Mixture should be marbled with thick ribbons of nut butter. Spoon heaping spoonfuls of meringue onto parchment-lined baking sheets (meringues will flatten slightly as they bake, so make them tall) and top with coffee beans. Bake until dry, 22 1/2 hours. Turn off heat and let cool in oven. Do ahead: Meringues can be made 2 days ahead. Store airtight at room temperature.