In a small bowl, combine flour, cocoa, and baking soda.
In a large mixing bowl, beat the butter, oil, and sugar until smooth.
Add eggs, beating well after each egg.
Add vanilla.
Alternately add the dry ingredients and buttermilk until completely mixed in. Fold in zucchini.
Pour into a greased 13 x 9 baking pan.
Bake at 325 for 45 - 50 minutes or until an inserted toothpick comes out clean.
Cool on a wire rack.
Notes
You can make your own buttermilk by adding 1 Tablespoon lemon juice to 1 cup milk. Let it stand for 5 minutes, then use as you would use real buttermilk.