1. Beat butter and peanut butter at medium speed with an electric mixer until blended; gradually add sugar, beating well. Add eggs, one at a time, beating after each addition.
2. Combine flour, baking powder and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Mix at slow speed after each addition until blend. Stir in vanilla.
3. Pour batter into a greased and floured 13 x 9-inch pan. Bake at 350 degrees F. for 50 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack.
4. Spread chocolate frosting on top of cake and sprinkle with peanuts. Cut into squares. Store in refrigerator.