Preheat oven to 350°F (175°C). Line an 8x8 or 9x9 inch baking pan with parchment. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

In a large bowl, whisk the eggs and granulated sugar until pale and slightly thickened, about 1 minute. Whisk in the oil, sour cream, and vanilla until smooth.

Pour the dry ingredients into the wet mixture and stir until just combined. Fold in the grated zucchini until evenly distributed.

In a small bowl, combine the streusel flour, brown sugar, and cinnamon. Cut in the cold butter with your fingertips until the mixture forms pea-sized crumbs.

Scrape the batter into the prepared pan and smooth the top. Sprinkle the streusel evenly over the batter, pressing lightly. Bake for 50 to 55 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Cool the cake completely in the pan on a wire rack, about 1 hour. Lift out using the parchment, slice, and serve.