In a medium bowl, mix together the flour, cornstarch, brown sugar, sugar, and cinnamon.

Pour in the butter and mix, ensuring all of the crumble is evenly hydrated with no patchy dry spots. It should be very thick and clumpy.

Place the bowl in the refrigerator while we make the cake batter.

Preheat the oven to 350F and arrange the oven rack in the middle of the oven. Grease and line an 8×8 pan with parchment paper. Allow the paper to reach up and over the sides for easy removal once the cake is baked.

In a mixing bowl using a hand or stand mixer with the paddle attachment, add the butter and ½ cup of sugar. Mix on medium-high speed until creamy, light, and fluffy.

Pour in the remaining ¼ cup of sugar and mix to combine.

Now mix in one egg, scrape down the bowl, and mix in the other egg along with the vanilla.

Once smooth, mix in the sour cream followed by the baking soda and salt.

Last, mix in the flour. When it appears to be fully mixed in, switch to a rubber spatula to gently mix the batter a couple times by hand just to ensure everything is evenly combined.

Spread the batter into the prepared pan. Taking the crumble from the refrigerator, sprinkle an even layer over the cake batter and ensure absolutely no cake batter is peaking through.

Bake for 50-55 minutes or until the cake has an even rise with golden brown edges.

Allow the cake to cool for 30-45 minutes, then release it from the pan.

Top with a dusting of powdered sugar, slice, and enjoy!