• 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, 1/2 tsp cinnamon

For the streusel

• In a medium sized bowl, add flour, brown sugar, white sugar, cinnamon, and melted butter (make sure the butter is not hot). 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. You may need 1 more tbsp of flour if the butter hasn't been completely absorbed. Set aside in the fridge when done.
  1/4 cup white sugar, 1/4 cup brown sugar, 3/4 cups all-purpose flour + 1-2 tbsp on the side, 6 tbsp butter, 1/2 tsp cinnamon

For the coffee 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 teaspoon baking soda, 1 teaspoon baking powder, 1/2 tsp salt, 1 tbsp cornstarch
• In a large bowl, add white sugar and oil and whisk together. Then add in eggs, applesauce, sour cream, and vanilla. Whisk together until well combined.
  1/2 cup oil, 3/4 cups white sugar, 1/2 cup sour cream, 1 cup unsweetened applesauce, 2 eggs, 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 spread evenly, and then the rest of your streusel on top.
• Bake the cake for 35-50 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.