This is the mousse recipe that goes inside our Chocolate Roulade with White-Chocolate Mousse and Fudge Frosting.
Dissolve gelatin in 1/4 cup cold water in a small bowl. Let stand 5 minutes.
Pulse white chocolate in a food processor until finely chopped. Bring 3/4 cup cream just to a boil in a small saucepan over medium-high heat. Add gelatin mixture; stir to dissolve, about 30 seconds.
With machine running, add cream mixture to white chocolate; process until smooth. Transfer mixture to a medium bowl, cover with plastic wrap; chill until thick enough to hold a ribbon on surface when drizzled, about 20 minutes.
Beat remaining 1 1/4 cups cream until stiff peaks form. Fold into white-chocolate mixture. Chill until almost firm and set, 25 to 30 minutes. Whisk mousse until smooth, and use immediately.