In a large bowl, add your sugar, oil, vanilla extract and sour cream and whisk until well combined.
Using a clean dish cloth or a paper towel, soak the moisture out of your pumpkin puree, patting it dry. Add your pumpkin puree to the mixture and stir again.
Add your eggs one at a time, mixing in between each addition.
In a medium bowl, stir together your dry ingredients, including your cinnamon, flour, cornstarch, baking soda, baking powder and salt.
Add ⅓ of your dry ingredients into your mixture and mix until just combined. Add half your milk.
Add an additional ⅓ of the dry Ingredients and mix and you second half of milk and mix.
Lastly, add you last ⅓ of dry ingredients and mix until just combined, being careful to not over mix.
Preheat your oven to 350°F
For the streusel, add your cold butter, oats, brown sugar, cinnamon and flour and use a fork, your hands or a pastry cutter to blend until it starts to come together. It’s ok if it’s still a little crumbly.
Add parchment paper to your 9 by 9 square baking dish and pour in half your batter. Take half your crumble mixture and crumble it on top of the batter.
Add the rest of the batter on top.
Crumble the rest of your crumble on top of your cake and bake in the oven at 350° for 40-45 minutes until a knife comes out clean. (If at 45 minutes, your cake is still sticky and wet, keep it in the oven for 5 minutes at a time checking until it’s done)
To make the glaze, add your powdered sugar and milk and whisk until smooth.
Once your cake has cooled, cut them into slices 4 by 4 and drizzle your glaze over the top.
Share and enjoy!!