Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners. Shred 2 cups zucchini, then squeeze firmly in a towel until you have about 1 ½ packed cups.

Whisk together the 1 ½ cups flour, baking soda, 1 tsp cinnamon, and salt in a large bowl.

Beat the eggs, oil, granulated sugar, brown sugar, and vanilla in a medium bowl until smooth. Stir in the squeezed zucchini.

Fold the wet ingredients into the dry just until no flour streaks remain. Gently fold in the walnuts.

Make the streusel by rubbing the remaining flour, brown sugar, cinnamon, and butter together until crumbly. Divide the batter among the muffin cups and top with streusel, pressing it lightly.

Bake 18 to 22 minutes until a toothpick comes out clean. Cool in the pan 5 minutes, then transfer to a wire rack. Serve warm or at room temperature.