Beat together first 5 ingredients and rum, if desired, in a large mixing bowl at low speed with an electric mixer until blended. Add cake mix, beat at medium speed 2 minutes. Pour batter into a greased and floured Bundt pan.
Bake at 350° for 50 minutes or until a toothpick inserted in center comes out clean. Cool in pan on a wire rack 15 minutes; remove cake from pan and cool completely on wire rack.
Microwave frosting on high 15 to 20 seconds or until pourable. Let stand one minute before drizzling over cake. Sprinkle pecans over frosting.