Heat hazelnut spread in metal bowl over saucepan of gently simmering water and stir until hazelnut spread is warm, smooth and easily stirred, about 5 minutes. Remove bowl from heat. Dissolve sugar in water in a small saucepan over medium heat. Scoop caramel mixture with a melon baller into 1 tablespoon portions; roll in the cocoa powder to coat; arrange onto a baking tray. Lift truffles from chocolate with a fork and tap fork against side of bowl to remove excess coating.