The gelatin in the mousse stabilizes it, so you can make and fill the roll a day ahead without worrying about it weeping or deflating.
- ¾ teaspoon unflavored powdered gelatin
- 6 ounces vacuum-packed or jarred roasted chestnuts
- ½ teaspoon vanilla extract
Place 1 Tbsp. cold water in a small bowl and sprinkle gelatin over; set aside.
Meanwhile, bring chestnuts, milk, salt, and vanilla to a simmer in a small saucepan and cook until chestnuts are falling apart and milk has reduced by half, 12−18 minutes. Add gelatin, stirring to dissolve. Transfer mixture to a food processor and blend until very smooth. Let chestnut cream cool.
Whisk cream in a medium bowl to soft peaks. Using an electric mixer, beat yolks and granulated sugar in another medium bowl until pale and thick, about 4 minutes. Beat in cooled chestnut cream, then use a spatula to gently fold in whipped cream.