Preheat oven to 350°. In a small bowl, mix hazelnuts and sugar. In a large bowl, whisk flour, confectioners' sugar and cornstarch. Cut in butter until crumbly. Transfer to a clean work surface. Knead gently until mixture forms a smooth dough, about 2 minutes (dough will be crumbly but will come together).
Divide dough in half. On a lightly floured surface, roll each portion to 1/8-in. thickness. Cut with a floured 2-in. round cookie cutter. Place 1/4 teaspoon Nutella in centre. Fold dough partially in half, just enough to cover filling.
Place 1 in. apart on greased baking sheets. Brush with beaten egg; sprinkle with hazelnut mixture. Bake 10-12 minutes or until bottoms are light brown. Remove from pans to wire racks to cool.