Preparation

To make the mousse:

• In a 2-qt. metal bowl set over a saucepan of simmering water, melt the chocolate, stirring with a rubber spatula until the chocolate is completely smooth. In a separate small saucepan, heat 1/2 cup of 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 hot cream into the melted chocolate and stir together with the spatula until well blended. Add the hazelnut butter and stir until well combined. Stir in the 2 tsp. vanilla.
• In a chilled mixing bowl, using chilled beaters, beat the remaining 1 cup cream until it holds soft peaks. With a rubber spatula, fold the whipped cream into the chocolate mixture in four batches, blending thoroughly after each addition. Pour the mousse into a 1-1/2 qt. soufflé dish or serving bowl, or into individual serving bowls or glasses. Cover with plastic wrap and refrigerate until set, at least 2 hours (or 1 hour if using it to fill profiteroles).
  

To make the garnish:

• In a chilled mixing bowl with chilled beaters, beat the 1/2 cup cream until frothy. Add the 1/2 tsp. vanilla and continue beating until the cream holds soft peaks. Pipe or spoon the whipped cream on top of the mousse. Sprinkle the chopped hazlenuts over the whipped cream and serve.

Make Ahead Tips

This mousse is best when served within 6 hours of the time it’s made.