Preheat your oven to 350 degrees F. Grease a 9×13 inch baking pan.
In a small saucepan, brown 8 tablespoons of unsalted butter over medium-low heat until it's golden brown and fragrant.
In a large bowl, whisk together 2 1/2 cups of all-purpose flour, 1 cup of light brown sugar, 1 teaspoon of baking soda, 1/2 teaspoon of salt, 1 1/2 teaspoons of ground cinnamon, and 1/4 teaspoon of ground nutmeg.
In another bowl, whisk together cooled browned butter, 1 cup of plain Greek yogurt, 2 large eggs, and 2 teaspoons of vanilla extract.
Pour the wet mixture over the dry ingredients and stir gently until combined. Then fold in 1 1/2 cups of shredded zucchini.
Transfer the batter into the prepared pan and spread evenly.
For the streusel topping, brown 4 tablespoons of unsalted butter until fragrant.
In a small bowl, combine browned butter, 1/2 cup of brown sugar, 1/2 cup of all-purpose flour, and 1/2 teaspoon of ground cinnamon until crumbly.
Sprinkle the streusel topping over the cake batter.
Bake for 30 to 35 minutes or until a toothpick inserted comes out clean.
Prepare the glaze with 1 cup of powdered sugar, 2 tablespoons of milk, 1/2 teaspoon of vanilla extract, and a pinch of ground cinnamon.
Once cooled, drizzle the glaze over the cake and cut into squares.
