Preheat oven to 350°F. Grease a 12-count muffin pan with nonstick spray or line with cupcake liners. Lightly grease the top of the pan to keep the muffin tops from sticking.
In a large bowl, whisk together the brown sugar, eggs, oil, and vanilla extract. Mix until smooth. Add the shredded zucchini. Mix to combine. Add the flour, baking powder, baking soda, cinnamon, and salt. Mix until a batter forms. Stir in nuts, dried fruit, or chocolate if using.
Allow batter to rest for five minutes while you prepare the crumb topping.
Stir together the flour, oats, cinnamon, and salt. Add the brown sugar. Break up any lumps with a fork or your fingers. Stir in the melted butter until the mixture is crumbly.
Spoon the batter into prepared muffin cups, filling them about ⅔ full. Sprinkle crumb topping, about two teaspoons each, onto each muffin.
Zucchini Bread. Grease a 9x5-inch loaf pan. Spread batter into pan. Top with crumb topping. Bake until a cake tester inserted in the middle comes out clean, about 50 minutes.
Zucchini Bundt. Double the recipe. Spread half the batter into a greased 10 or 12 cup Bundt pan. Sprinkle half the crumb mixture over the batter. Repeat with remaining batter and crumb topping. Bake until brown and a cake tester comes out clean, about 80 minutes.