Combine egg yolks, lemon juice, 1/2 cup sugar, and salt in top of a double boiler. Cook over boiling water, stirring constantly, until thickened. Add softened gelatin, lemon rind, and lemon extract; stir well. Continue to cook over boiling water, stirring until gelatin dissolves. Cool completely. Transfer to a large mixing bowl.
Beat egg whites (at room temperature) until foamy. Gradually add remaining sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold into lemon mixture.
Beat whipping cream and vanilla until soft peaks form. Fold into lemon mixture. Spoon evenly into parfait glasses; cover and chill. Garnish with lemon slices.