In a metal bowl set over a saucepan of simmering water, melt the bittersweet and milk chocolate, stirring frequently with a rubber spatula to ensure even melting. In a small saucepan, heat the cream over medium heat to just below the boiling point.

Remove the bowl of chocolate from the pan of water and wipe the bottom and sides dry. Pour the cream into the chocolate and stir with a spatula until very smooth. Stir in the hazelnut butter until well blended. Cover tightly with plastic wrap. Cool to room temperature, and then refrigerate until firm, about 3 hours, stirring occasionally with the spatula.

With a small spoon or melon baller, scoop 3/4-inch balls of the chocolate mixture and roll them between your palms to round them off. Place the rolled truffles on a parchment-lined sheet tray.

Sift the cocoa into a shallow bowl. Drop the truffles, a few at a time, into the bowl and shake and roll them in the cocoa until well coated. Transfer the truffles to a container with a tight-fitting lid, making sure they don't touch each other. Refrigerate until ready to serve. Sealed, refrigerated truffles will keep for up to a week.