Preheat oven to 325°. Spray a 9-inch round cake pan with nonstick cooking spray. Line pan with parchment paper, letting edges of parchment hang over sides of pan.

In a medium saucepan, combine butter, chocolate, and peanut butter over medium-low heat. Cook, stirring frequently, until chocolate melts and mixture is smooth. Remove from heat.

Whisk sugar into butter mixture, whisking for 2 minutes. Add egg yolks, one at a time, whisking well after each addition.

In a medium bowl, beat egg whites at high speed with a mixer until stiff peaks form. Gently fold egg whites into chocolate mixture. Spoon batter into prepared pan.

Bake for 1 hour. Let cool for 1 hour. Cover and chill for 4 hours. Serve with Peanut Butter Sauce.

PEANUT BUTTER SAUCE: In a medium saucepan, combine cream, brown sugar, and egg yolks. Bring to a boil over medium heat; reduce heat, and simmer for 6 to 8 minutes, or until thickened. Add peanut butter, whisking until smooth. Serve sauce warm.