Make the cookies: Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Spread the hazelnuts on a rimmed baking sheet and bake until golden brown, 9 to 11 minutes; let cool, then finely chop. Reduce the oven temperature to 225 degrees F and line 2 baking sheets with parchment paper.
Whisk the egg whites, cream of tartar and salt in a large bowl with a mixer on medium speed until soft peaks form, about 3 minutes. Increase the mixer speed to medium high and beat in the granulated sugar, 1 tablespoon at a time. Continue beating until stiff peaks form, 6 to 8 minutes. Mix in the vanilla. Add the chopped hazelnuts and sift in the confectioners' sugar.
Fold together with a rubber spatula.
Transfer the meringue to a pastry bag fitted with a large round tip. Pipe 2-inch rounds about 1 inch apart on the prepared pans, keeping the pastry tip close to the pan.
Bake, switching the pans halfway through, until the meringues are dry and release from
the parchment, 1 1/2 to 2 hours. Turn off the oven; leave the meringues inside for 30 minutes. Transfer to racks to cool completely.
Meanwhile, make the filling: Beat the butter, confectioners' sugar and salt in a bowl with a mixer on medium speed until combined, 1 minute. Increase the speed to medium high and beat in the vanilla; beat until fluffy, about 3 minutes. Dollop 1 teaspoon filling and 1/2 teaspoon jelly on the flat side of half the meringues. Top with the remaining meringues.