Mix the cornstarch, flour, cocoa powder, powdered sugar, vanilla sugar, and espresso powder. Add the butter and work into a smooth dough. Shape into 2 rolls (each about 15 cm long), cover, and refrigerate for 1 hour.
Cut the rolls into 25 slices each (50 total). Wrap each slice around a hazelnut and shape into a ball. Place on baking sheets lined with baking paper.
Bake at 200°C (392°F) for 12-15 minutes until set. Let cool on the baking sheets for 10 minutes.
Melt the couverture and hazelnut cake icing together, stirring gently until smooth.
Dip half of the cooled balls into the melted chocolate mixture. Place a mocha bean on top of each dipped ball and let dry for at least 20 minutes.