Heat oven to 375°F. Place paper baking cup in each of 12 regular size muffin cups.
In large bowl, stir flour, brown sugar, cocoa, baking soda, and salt with spoon or whisk.
In small bowl, beat buttermilk, melted butter, eggs and vanilla with whisk until blended.
Pour buttermilk mixture into flour mixture; stir until just moistened. Stir in zucchini, chocolate chips and nuts.
Divide batter evenly among muffin cups.
Bake 20-24 minutes or until toothpick inserted in center comes out clean.
Cool 5 minutes; remove from pan. Server warm or cool.