Preheat the oven to 350°F and grease a 9x5-inch loaf pan.
In a large mixing bowl, whisk together flour, sugars, baking powder, baking soda, salt, and cinnamon.
In another bowl, beat eggs, oil, milk, zucchini, and vanilla until smooth.
Pour the wet mixture into the dry mixture and stir until just combined. Don’t overmix.
Pour the batter into the prepared loaf pan.
For the streusel, combine flour, brown sugar, and cinnamon in a bowl. Cut in the butter until crumbly.
Sprinkle streusel over the batter.
Bake for 55 to 60 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes before transferring to a wire rack.