Preheat the oven to 325 and line an 8x8 baking dish with parchment paper with a couple inches hanging over two opposites sides to assist in easy removal of the cake.

To make the crumb topping add the flour, sugar, brown sugar, cinnamon, and salt to a bowl, whisk to combine. Add the melted butter and mix with a fork until the texture is crumbly.

To make the cake add the flour, sugar, baking soda, baking powder, and salt to a large bowl and whisk to combine.

Add the milk, oil, vinegar, and extract to a separate bowl and whisk to combine. Add the zucchini to the wet mixture and stir to combine. Add the wet zucchini mixture to the dry mixture and mix with a spatula until just combined, do not overmix.

Fill the prepared baking dish with batter and sprinkle the crumb topping over the top in one layer, covering the cake completely.

Bake for 45 minutes, until a toothpick inserted in the center comes out clean. Let cook completely.