Peanut Butter Filling: In bowl, beat together peanut butter, cream cheese and butter. Beat in egg yolks, 1 at a time, and vanilla. Beat in sugar and flour. Scrape into piping bag fitted with 1/2-inch (1 cm) plain tip. Place upright in tall container and set aside in refrigerator.
In heatproof bowl, whisk cocoa with boiling water; whisk in chocolate until smooth. Whisk in sour cream. Set aside.
In large bowl, beat butter with sugar until fluffy. Beat in eggs, 1 at a time, and vanilla. In separate bowl, whisk together flour, baking soda and salt. Stir into butter mixture alternately with cocoa mixture, making 3 additions of flour mixture and 2 of cocoa mixture.
Spoon a little less than half of the batter into bottom of prepared pan. Pipe ring of peanut butter mixture over centre of chocolate batter. Spoon remaining chocolate batter over top; tap pan lightly on counter to remove air bubbles.
Bake in centre of 350°F (180°C) oven until top is just firm to the touch and edge begins to pull away from side of pan, about 55 minutes. Let cool in pan on rack for 5 minutes. With knife, loosen cake. Place rack over pan and turn out cake. Let cool completely.
Whip cream. Slice cake into wedges and serve with dollop of whipped cream sprinkled with chopped peanuts.