Beat everything for filling in a mixer until creamy then chill in the fridge for half hour before using
Mix 2-3 tbsp of filling (reserve the rest for other pastries/cakes/chocolates/breads/sweets) with generous 1-2 tbsp Nutella until combined
Melt dark chocolate and fill up mold, tap upside down, chill in freezer for 5 minutes
Fill up cavities with Nutella mixture and pour melted chocolate
Chill until harden.