Beat together, in a large bowl, butter and sugar with an electric mixer on medium speed unti light and fluffy, 1 to 2 minutes.
Add peanut butter and melted chocolate and beat until well-blended.
Add eggs, one at a time, beating well after each addition.
Sift together flour, baking powder, and baking soda; beat into chocolate mixture alternately with milk, just until combined.
Stir in chocolate chips.
Turn batter into prepared pan.
Bake 55-60 minutes or until a cake tester inserted in center comes out clean.
Let cool in pan 15 minutes, then unmold onto a rack and let cool completely.
Frost cake all over with Quick Chocolate Frosting and sprinkle nuts on top.
Quick Chocolate Frosting: In a 1-quart glass bowl, combine chocolate chips and butter; heat in a microwave on high 1 to 1 1/2 minutes, or until melted and smooth when stirred; Beat in powdered sugar and milk.