Peanut Butter Cake with Chocolate-Peanut Butter Icing

Preheat oven to 350 °F. Grease and flour 2 9-inch-diameter cake pans with 1 3/4-inch-high sides. Prepare cake according to package directions, beating in 1/2 cup peanut butter. Divide between prepared pans. Bake until toothpick inserted into centers comes out clean, about 20 minutes. Cool cakes in pans on racks 10 minutes. Turn cakes out onto racks and cool completely. Combine chocolate syrup and semisweet chocolate in heavy small saucepan. Stir constantly over low heat until chocolate is melted and mixture is smooth. Transfer chocolate mixture to large bowl. Stir in remaining 2 cups peanut butter, then powdered sugar, then milk. Continue stirring until smooth. Place 1 cake layer on platter. Spread top with 1 1/2 cups icing. Top with second cake layer. Spread top and sides of cake evenly with remaining icing. Garnish with chopped peanuts. (Can be prepared up to 1 day ahead. Cover.) Cut into wedges and serve.