• Preheat the oven to 350℉/180℃. Line a 9x9 pan with parchment paper and set aside.

For the streusel

• In a bowl, add the butter, brown sugar, white sugar, flour, and cinnamon. With your hands of a fork, combine until a streusel crumb forms. Set in the fridge while making the batter.
  1 1/2 cups all-purpose flour, 1 1/2 tsps cinnamon, 1/2 cup white sugar, 1/3 cup dark brown sugar, 1/2 cup + 2 tbsp unsalted butter

For the cake

• In a bowl, add the flour, baking soda, baking powder, cornstarch, and salt. Stir to combine.
  1 3/4 cups all-purpose flour, 1/2 tsp baking powder, 1/2 tsp baking soda, 1 1/2 tsp cornstarch, 1/4 tsp salt, 3/4 cups white sugar
• Add in about 1/3 of the butter. On low speed, mix until just combined. Then add in the next 1/3 of butter, and mix well, scraping the bowl as needed. Then the last bit of butter, and mix until a crumbly texture forms (see image above for reference). The key to this cake is mixing as little as possible, so only mix each addition in until just combined!
  2/3 cups unsalted butter
• In a separate bowl, add the egg, egg yolk, oil, sour cream, and vanilla. Whisk until well combined.
  2 tbsp oil, 1 large egg, 1 large egg yolk, 3/4 cups sour cream, 1 tbsp vanilla
• Combine the butter/flour mixture with the wet ingredients. Using a mixer on low speed, mix until just combined. The batter will be very smooth.
• Transfer into the prepared pan and spread evenly. Sprinkle streusel on top.
• Bake for 35-45 minutes or until a toothpick inserted in the middle comes out clean. Allow to cool, slice, and enjoy!