Preheat the oven to 350 degrees F. Grease a 9x13 pan and set aside.

In a small saucepan, brown the butter for the cake. Melt the butter over medium-low heat and continue to cook, swirling occasionally, until butter turns golden brown and has a nutty aroma. Pour butter into a small bowl and set aside to cool to room temperature.

In a large bowl, whisk together flour, brown sugar, baking soda, salt, cinnamon, and nutmeg. Set aside.

In a small bowl, whisk together browned butter, Greek yogurt or sour cream, eggs, and vanilla extract.

Pour liquid ingredients over dry ingredients and stir. The batter will be very thick and somewhat dry. Add the shredded zucchini and stir until cake is combined and moistened. Again, it will be very thick. Pour cake batter into prepared pan.

To make the streusel topping, brown the butter. Melt the butter over medium-low heat and continue to cook, swirling occasionally, until butter turns golden brown and has a nutty aroma. Pour butter into a small bowl and set aside to cool to room temperature. In a small bowl, combined browned butter, brown sugar, flour, and cinnamon. Mix together until crumbly. Sprinkle topping evenly over cake.

Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean and the cake is light golden brown. Let the cake cool on a wire cooling rack.

While the cake is cooling, make the glaze. In a small bowl, whisk together the powdered sugar, milk, vanilla, and cinnamon. Whisk until smooth and drizzle over cooled cake. Cut into squares and serve.