POSITION oven racks in top third and centre of oven. Preheat oven to 200F. Line 2 baking sheets with parchment. Stir sugar and cornstarch in a small bowl. Beat egg whites with cream of tartar in the bowl of a stand mixer on medium-high until soft peaks form when beaters are lifted, about 2 min. Gradually beat in sugar mixture, 2 tbsp at a time, until stiff and glossy peaks form when beaters are lifted, about 3 min. Fold in ¾ cup finely ground hazelnuts when stiff peaks form.
FIT a piping bag with a small plain tip. Pipe into 1 ½-in. rounds about ½ in. high.
BAKE in centre and bottom third of oven, switching and rotating sheets halfway through, until tops are firm and bottoms are dry, about 2 hours. Turn off heat and let stand in oven for 1 hour more. Transfer to racks to cool completely. Spread the bottoms of half of the meringues with ½ tsp chocolate-hazelnut spread each and sandwich with remaining meringues. Meringues will keep well in an airtight container at room temperature for up to 1 week. Do not refrigerate or freeze. Baking tip: When working with meringue, pipe and bake immediately. Meringue will lose volume if left too long.