Add the flour, cornstarch, brown sugar, sugar, cinnamon, and sprinkles to a medium bowl. Pour the melted butter into the bowl and mix thoropughly with a fork to form a thick and clumpy crumble. Place bowl in the refrigerator while making the cake batter.

Preheat the oven to 350F.

Line an 8×8 pan with parchment paper, making sure that you create a parchment paper sling that hangs over the sides of the pan for easy removal after baking.

In a small bowl, add flour, baking soda, sprinkles and salt. Mix with a fork to combine and set aside.

In a medium bowl with a hand mixer, or the bowl of a stand mixer fitted with a paddle attachment, add the butter and the granulated sugar. Mix on medium-high speed for about 4 minutes in a stand mixer, and at least 6 with a hand mixer, mixture should be light and fluffy.

Add in both eggs and vanilla, scraping down the stand mixer paddle and the sides of the bowl to make sure everything is incorporated evenly.

Next, add in the sour cream and mix again. Scrape down the sides of the bowl and mix in the flour, baking soda, sprinkles and salt until just combined.

Pour batter into prepared pan and smooth the top with an offset spatula.

Sprinkle the crumble on top of the batter in an even layer.

Bake for 45-55 minutes or until a toothpick comes out clean.

Allow cake to cool for at least an hour.

In a small bowl, combine all ingredients. Mix with a spoon to form a thick icing. It should be smooth and slowly falls from the spoon. Drizzle on top of cooled cake and top with more sprinkles before slicing.