Heat oven to 350°F. Grease and flour 12-cup Bundt® pan. In large bowl, combine all cake ingredients; beat at low speed until moistened. Beat 2 minutes at medium speed. Pour batter into greased and floured pan.
Bake at 350°F. for 45 to 55 minutes or until toothpick inserted in center of cake comes out clean. Cake will be moist on top. Cool in pan for 10 minutes. Remove from pan; place on wire rack. Cool 45 minutes or until completely cooled.
Place cooled cake on serving platter. Place frosting in small microwave-safe bowl. Microwave on HIGH for 25 to 35 seconds or until of thin consistency. Immediately stir in peanut butter. Drizzle warm frosting over top of cake, allowing some to drizzle down sides.