Preheat oven to 350 degrees F. Lightly coat a 8x8” (or 9x9” also works) pan with parchment paper (or you can also spray with cooking spray or coat with oil).
In a small bowl, make the crumb topping by first adding flour, oats, brown sugar and cinnamon to the bowl. Combine using a whisk or fork.
Add butter to the dry ingredients and mix well using your hands until fully combined and crumbly (should resemble wet sand). Make sure to fully work the butter into the dry ingredients.
In a large bowl, add all wet ingredients: pumpkin puree, Greek yogurt, milk, eggs, avocado oil, brown sugar, and vanilla. Whisk until fully combined and smooth.
Next, add all dry ingredients to a medium bowl and whisk to combine.
Add dry ingredients to wet ingredients and mix together until just combined.
Pour batter into baking pan and then top evenly with the topping.
Bake for 28-30 minutes, or until a toothpick inserted into the center comes out clean (or with just a crumb or two). Note: If you’re not using a metal baking pan, you will need to add a few minutes onto your bake time.
(Optional) In a small bowl prepare the glaze by mixing sugar and milk. You can add more sugar to make it thicker or more milk to make it thinner.
Allow to cool for 10 minutes, and slice. Drizzle with glaze, leave it plain or sprinkle with a light dusting of powdered sugar.