20 minutes, plus chilling time
1 cup chestnut purée, firmly packed
1/2 cup (1 stick) butter, softened
1 teaspoon vanilla extract or Amaretto
1 large egg white
1/2 cup sugar
Shaved chocolate or whipped cream for garnish
1. In a medium bowl,
mix together the chestnut purée, butter, vanilla extract and 2 tablespoons water with a hand mixer until smooth. Set aside.
2. Set a saucepan of water
over medium heat and bring to a simmer. Place the egg white in a deep metal bowl that fits over the saucepan without touching the water. Before setting the bowl over the simmering water, beat the egg white with a hand mixer on medium speed just until foamy, about 1 minute. Set the bowl over the simmering water, continuing to beat with the hand mixer (you don't want to let the egg white cook); reduce the heat to medium-low. Slowly add the sugar and beat on high speed until stiff, glossy peaks form, about 6 minutes.
3. Immediately fold the meringue
into the chestnut purée mixture. Chill for an hour, and up to 3 hours.
4. Spoon the mousse
into a pastry tube fitted with a large star tip. Pipe the mousse into glasses or bowls. Garnish with shaved chocolate or whipped cream.