1. Preheat oven to 350°. Beat butter and sugar in a large bowl with a stand mixer until well blended, or whirl in a food processor. Add cornstarch and flour and beat on low speed, or whirl, until dough comes together and is very smooth.
2. Divide dough in half. Roll each portion between sheets of parchment paper into a 1/8-in.-thick round. Stack and freeze rounds on a baking sheet until dough is firm enough to cut, about 20 minutes.
3. Lift off parchment, cut dough with a floured 1 1/2- to 2-in.-wide cutter, and transfer to ungreased baking sheets, spacing slightly apart. Gather scraps, refreeze, and reroll as needed.
4. Bake cookies until light golden, 10 to 14 minutes, switching pan positions halfway through baking. Transfer to racks to cool.
5. Spread half of cookies on flat sides with a generous coat of chocolate hazelnut spread, then top each with a second cookie.
Make ahead: Up to 3 days, stored airtight.
Note: Nutritional analaysis is per 1 1/2-in. sandwich cookie.