Preheat the oven to 350°F (175°C). Grease and flour a 9×9-inch baking pan.

Brown the butter by melting it in a medium saucepan over medium heat, swirling occasionally until it turns golden and smells nutty, about 5–7 minutes. Remove from heat and let cool slightly.

In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg.

In a large bowl, combine the granulated sugar, brown sugar, and browned butter until well blended. Beat in the eggs one at a time. Stir in the vanilla extract and sour cream until smooth.

Gradually add the dry ingredients to the wet mixture, stirring just until combined. Fold in the grated zucchini.

Spread the batter evenly into the prepared pan.

For the streusel, mix the flour, brown sugar, and cinnamon in a bowl. Add the cold butter and use your fingers or a pastry cutter to work it in until coarse crumbs form. Sprinkle evenly over the batter.

Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean. Let cool on a wire rack.

For the glaze, whisk the powdered sugar, milk, and vanilla until smooth. Drizzle over the cooled cake.

Slice into squares and serve.