Preheat oven to 350˚F. Combine 1 cup butter and 5 tablespoons cocoa in a saucepan over low heat; cook, stirring frequently, until butter melts. Remove from heat, and cool slightly.
Combine granulated sugar, flour, and salt in a large mixing bowl. Add chocolate mixture, and beat at medium speed with an electric mixer until blended. Add eggs and vanilla, mixing well. Spread into a well-greased 13- x 9- x 2-inch pan.
Bake in preheated oven for 20 to 25 minutes or until a wooden pick inserted in center comes out clean.
Transfer peanut butter from jar to a microwave-safe bowl. Heat on MEDIUM 2 to 3 minutes, or until peanut butter melts, stirring at 1-minute intervals. Spread over warm brownies. Chill about 30 minutes or until set.
Combine remaining 1/2 cup butter, remaining 4 tablespoons cocoa, and milk in a saucepan over medium heat; cook, stirring frequently, until marshmallows melt. Remove from heat, and add powdered sugar, stirring until smooth. Spread over peanut butter layer, and chill until set, about 30 minutes. Store in refrigerator.