These bars are a grown-up spin on a campfire classic: buttery graham cracker crust, chocolate mousse, toasty meringue.
Preheat oven to 325°. Line a 9-inch square baking pan with two lengths of foil, crisscrossed, leaving some overhang. In a food processor, pulse crackers, butter and 3 tablespoons sugar to the consistency of damp sand. Pat firmly into bottom of lined pan. Bake until golden and fragrant, about 15 minutes. Cool on a wire rack.
Break up semisweet and unsweetened chocolate and place in a microwave-safe bowl. Zap in 30-second intervals, stirring after each, until just melted. Let cool.
In the bowl of a stand mixer fitted with whisk attachment, whip cream to stiff peaks. Transfer to another bowl and chill. Wash and dry mixer bowl and the whisk.
In the stand mixer bowl, combine egg whites, 1 3/4 cups sugar, the cream of tartar and salt. Set bowl over a pan of simmering water. Whisk by hand constantly until mixture reaches 165°, about 4 minutes. (Start checking temperature after 3 minutes.) Lift out bowl, dry the bottom and snap into mixer. Whisk on high speed to glossy, cool, stiff peaks, about 4 minutes. Add vanilla and beat 1 minute more to return to stiff peaks. Transfer half of this meringue to a separate bowl; set aside.
Gently fold cooled chocolate and the rum, if using, into remaining egg white mixture. Fold in whipped cream. Spread mixture over cooled crust. Top with reserved meringue, swirling with the back of a spoon.
Preheat broiler. Broil 4 to 5 inches from the heat 30 seconds to 1 minute. (Watch closely so it doesn't burn.) Chill at least 2 hours. To serve, grasp foil overhang and lift out onto a cutting board. Slice into squares.