This lemony mousse fills our Citrus Mousse Cake with Buttercream Frosting.
Sprinkle gelatin over 2 tablespoons cold water; let stand until soft, about 5 minutes. Meanwhile, in a medium saucepan off heat, whisk together 1 cup granulated sugar, zest, and egg yolks. Whisk in lime and lemon juices and salt. Add butter; place over medium-high heat. Cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around edge of pan, about 5 minutes. Stir in gelatin mixture; cook until completely dissolved, about 1 minute more.
Pour curd through a fine-mesh sieve into a nonreactive bowl. Press plastic wrap against surface of curd; refrigerate until cold, about 1 hour (or stir over an ice bath until chilled, about 15 minutes).
3. Whisk cream with confectioners' sugar until stiff peaks form. Whisk curd briefly to loosen, then fold in whipped cream. Beat egg whites with whisk attachment until soft peaks form, then gradually beat in remaining 1/4 cup granulated sugar until stiff peaks form. Gently fold into curd mixture. Refrigerate in an airtight container at least 1 hour and up to 2 days.