Preheat oven to 350°F. Butter two standard 12-cup muffin pans or line pans with paper liners.
Combine ½ cup each of flour and brown sugar with cinnamon in a small bowl. Blend in ¼ cup butter with fork until mixture resembles coarse crumbs. Stir in nuts and set streusel mixture aside.
In a large bowl, whisk together flour, cocoa, salt, baking soda, baking powder. Set aside.
In a mixing bowl, cream butter and sugar until light and fluffy.
Beat in eggs.
Blend in flour mixture along with zucchini and vanilla.
Divide mixture between prepared muffin cups (about ¼ cup per each cup) and stop with streusel mixture (about 1 Tablespoon per each muffin.)
Bake for 20-25 minutes or until center is firm and tests done.