Preheat the oven to 350°F and prepare a 9x5 loaf pan by greasing it and lining it with parchment paper.

Grate about 2 cups of zucchini (around 2 medium zucchinis), without packing it tightly.

In a large bowl, whisk together melted butter, eggs, sugar, and vanilla until well combined. Add flour, baking powder, baking soda, and salt. Mix until mostly combined, then fold in the grated zucchini and any optional add-ins like nuts or chocolate chips until no dry spots remain.

Pour the batter into the prepared pan.

In a separate bowl, mix the streusel dry ingredients, then drizzle in melted butter. Use your fingers to squeeze the mixture into clumps. Sprinkle the streusel over the batter.

Bake for 50 to 60 minutes, until a toothpick inserted in the center comes out clean.

Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.