Preheat oven to 325 degrees F (162 degrees C). Grease and flour pan; set aside. In large bowl, combine butter, sugar and brown sugar; beat on medium speed until light and fluffy. Stir in vanilla. Add eggs, one at a time, beating well after each addition. In medium bowl, sift together flour, cocoa, baking soda and salt. On low speed, add flour mixture to butter mixture in three additions, alternating with buttermilk and beginning and ending with flour. Add sour cream. Beat on medium speed 1 minute, scraping bowl often. Fold in chopped chocolate. Spoon batter into prepared pan.
Peanut Butter Filling: In medium bowl, combine peanut butter and powdered sugar; mix well. Spoon over batter, making sure filling does not touch sides of pan. Bake for 40-45 minutes, until toothpick inserted comes out clean. Cool in pan 15 minutes. Invert cake onto cooling rack to cool completely.
Glaze: In medium saucepan, combine chocolate chips and whipping cream. Cook on low heat, stirring constantly, until smooth and melted. Remove from heat and cool slightly. Place cake on serving platter and spoon glaze over cake. Garnish with chocolate curls and peanuts. Serve with whipped cream if desired. About 12 servings.