1. Preheat the oven to 175°C (350°F).

2. Spread the hazelnuts on a baking sheet and toast for 10-12 minutes until fragrant and lightly browned. Let cool completely.

3. Whisk the sugar and egg whites in a bowl until stiff peaks form.

4. In another bowl, melt the dark chocolate in the microwave using 30-second bursts, stirring between each until smooth.

5. Gently fold the egg white mixture into the melted chocolate until fully incorporated.

6. Chop the cooled, toasted hazelnuts and fold them into the chocolate mixture.

7. Cover and refrigerate for at least 30 minutes or until firm enough to shape.

8. Use a spoon or melon baller to scoop portions of the mixture and roll into 2.5 cm (1 inch) balls.

9. Place the balls on a parchment-lined baking sheet and chill for 10-15 minutes.

10. Roll each ball between your palms to form smooth spheres.

11. Gently press a whole hazelnut into the top of each truffle.

12. Chill the truffles for at least 30 minutes before serving. Store leftovers in an airtight container in the refrigerator for up to 5 days.