Preheat oven to 350 degrees F. Grease a 9 inch springform pan.
Make the streusel by adding the flour, brown sugar, and cinnamon to a bowl and mix with a fork. Add the melted butter and mix until a crumble forms. Place in the fridge until ready to use.
In a large bowl, make flax eggs by combining the ground flaxseed and water. Set aside for 5-10 minutes to thicken.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
In the large bowl with the flax egg, add the zucchini, pure maple syrup, coconut sugar, yogurt, oil, and vanilla extract. Whisk until well combined.
Add the dry ingredients to the wet ingredients and mix with a spatula until just combined. Be careful NOT to overmix.
Pour batter into prepared pan and evenly top with the streusel/crumb. Bake for 45 minutes until toothpick comes out clean. Let it cool in the pan on a wire rack.
Make the glaze by adding the powdered sugar, cinnamon, almond milk, and vanilla to a bowl. Whisk until smooth and thick. Drizzle over the cake. Sprinkle flaky sea salt on top. Enjoy! Store leftovers in an airtight container on the counter for 3 days. It tastes even better the next day as the flavors develop.