1. Preheat oven to 350°. Beat first 3 ingredients at medium speed with a heavy-duty electric stand mixer until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Sift together flour and next 4 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir zucchini and next 2 ingredients into batter until blended. Spoon batter into lightly greased loaf pans, filling two-thirds full.
2. Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pans on wire racks (about 1 hour).
3. Prepare Chocolate Fudge Frosting. Spoon hot frosting over cooled cakes (about 1/4 cup each); cool completely (about 30 minutes).