Preheat oven to 350 degrees F. Butter the side of a 2-quart souffle dish. For a collar on the souffle dish, measure enough parchment or foil to wrap around the top of the dish and add 3 inches. Fold the parchment into thirds lengthwise. Lightly grease one side of the parchment with butter; sprinkle with sugar. Place parchment, sugar side in, around the outside of the dish so parchment extends about 2 inches above the dish. Tape ends of parchment together. Sprinkle inside of dish with sugar; set aside.
In a small saucepan cook and stir chocolate over low heat until melted; set aside.
In a medium saucepan melt the 3 tablespoons butter. Stir in flour; gradually stir in half-and-half. Cook and stir over medium heat until thickened and bubbly. Remove from heat. Stir half of the hot cream mixture into the melted chocolate; set aside.
Stir peanut butter into the remaining hot cream mixture in the saucepan. In a medium bowl beat two of the egg yolks with a fork just until combined. Gradually stir the peanut butter mixture into the egg yolks; set aside.
In another medium bowl lightly beat the remaining two egg yolks. Gradually stir the chocolate mixture into the egg yolks; set aside.
In a large bowl beat egg whites with an electric mixer on medium to high speed until soft peaks form (tips curl). Gradually add the 1/3 cup sugar, 1 tablespoon at a time, beating until stiff peaks form (tips stand straight).
Gently fold half of the beaten egg whites into the peanut butter mixture. Gently spoon peanut butter mixture into the prepared souffle dish. Gently fold the remaining beaten egg whites into the chocolate mixture. Spoon chocolate mixture over peanut butter mixture in souffle dish. Using a table knife or thin spatula, gently swirl chocolate mixture into peanut butter mixture to marble.
Bake, uncovered, for 45 to 50 minutes or until a knife inserted near the center comes out clean. Serve souffle immediately. To serve, insert two forks back to back; gently pull souffle apart into six equal-size wedges. Use a spoon to transfer to plates. If desired, top with whipped cream.