In a large mixing bowl, beat the softened butter with a hand mixer until light and fluffy.

Add the powdered sugar, vanilla extract, and salt. Continue beating until well combined.

Add the flour, cocoa powder, and cornstarch. Mix on low speed until the ingredients come together to form a smooth, soft, buttery dough.

Divide the dough into two equal portions and flatten each portion into a disc.

Wrap each portion separately in plastic wrap and refrigerate until firm.

Preheat the oven to 150°C (300°F), using the fan-assisted setting. Line two shallow baking trays with parchment paper.

Place each portion of dough between two sheets of parchment paper. Lightly dust with cocoa powder and roll out into a thin sheet, approximately ½ cm (¼ inch) thick.

Refrigerate the rolled-out dough briefly, as it softens quite quickly.

Gather any leftover dough, bring it together, roll it out again, and cut out additional cookies.

Arrange the cookies on the prepared baking trays and bake one tray at a time for approximately 13 minutes.

Allow the cookies to cool completely before assembling. Pair them up and sandwich them together with a generous layer of hazelnut spread.