Heat oven to 350 degrees F. Coat 13x9-inch cake pan lightly with no-stick cooking spray.
Stir together flour, baking soda, salt and cinnamon in a medium bowl. Beat oil, peanut butter, sugar, eggs and milk in a large mixing bowl. Stir in carrots, walnuts, coconut and pineapple.
Add flour mixture and stir until combined.
Spread evenly in prepared pan.
Bake 32 to 37 minutes or until a toothpick inserted in center comes out clean. Cool completely on wire rack.
Stir frosting and peanut butter until combined.
Spread on cooled cake.
Place 3 tablespoons peanut butter in a resealable plastic bag. Microwave on HIGH 10 seconds. Make a small cut in one corner of bag.
Drizzle peanut butter over top of cake in lines across the short side. Then using a table knife, lightly slice through peanut butter down the long side to create design.