• Preheat the oven to 350°F/175°C and line and grease a 9×9 inch square pan. Combine brown sugar and cinnamon in a small bowl and set aside. This will be the filling.
  1/4 cup dark brown sugar, 1 tsp cinnamon

For the streusel

• In a medium sized bowl, add flour, brown sugar, white sugar, cinnamon, and softened butter. Using a fork, mix this until combined. Then add 1 tbsp of extra flour on top and using your hands, crumble the streusel into smaller and larger pieces. Set aside in the fridge when done.
  1/4 cup white sugar, 1/4 cup dark brown sugar, 1/2 tsp cinnamon, 3/4 cups all-purpose flour, 6 tbsp butter

For the cake

• In a medium-sized bowl, add in flour, baking soda, baking powder, cornstarch, and salt. Mix together and set aside.
  2 cups all-purpose flour, 1 tsp baking powder, 1 tsp baking soda, 1 tbsp cornstarch, 1/2 tsp salt
• In a large bowl, add butter, oil, eggs, sour cream, and vanilla. Whisk together until well combined. Then add in white sugar and whisk in.
  1/2 cup butter, 2 tbsp oil, 1 cup white sugar, 2 eggs, 1 cup sour cream, 1 tbsp vanilla
• Pour the dry ingredients into the wet and using a spatula, gently fold to create a soft cake batter.
• Add half of the batter into your prepared pan and spread evenly. Then sprinkle all of the cinnamon sugar topping, and about 4-5 tbsp of the streusel. Add the remaining half of your batter (I find using a cookie scooper works best to not disturb the layer below) and *gently* spread evenly, and then the rest of your streusel on top. Check photos above to see visual.
• Bake the cake for 35-55 minutes, or until a toothpick inserted comes out dry. Mine took 50 minutes. If it is wet, then the cake hasn’t fully baked yet so continue to bake at 5 minute intervals. Allow it to cool completely.