Preparation

• Put 3/4 cup of the cream in a medium metal bowl and refrigerate until ready to whip.
• Put the rum, bourbon, and vanilla in a small bowl, sprinkle with the gelatin, and let sit for 5 minutes to soften the gelatin. Bring the remaining 1/4 cup heavy cream, nutmeg, and salt to a simmer in a small saucepan. Remove from the heat and stir in the gelatin mixture to dissolve. Stir in the white chocolate. If necessary, return the pan to very low heat, stirring continuously, until melted and smooth. Transfer to a large bowl and whisk in the egg yolks. Refrigerate until cooled but not set, about 10 minutes.
• Whip the cold cream with a handheld electric mixer to stiff peaks, about 2 minutes. Fold the cream into the chocolate mixture until smooth but still streaky. Refrigerate while preparing the egg whites.
• Wash and dry the beaters thoroughly, and then whip the egg whites and cream of tartar in a medium bowl to stiff peaks, about 8 minutes. Fold the egg whites into the mousse until just incorporated.
• Divide the mousse among 5- to 6-oz. ramekins and refrigerate until chilled and firm, about 2 hours. Before serving, grate a generous amount of nutmeg over the top of the mousse and garnish with chocolate shards or curls, if using.