• 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. 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. Stir together and set aside to use later on.
  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. With your whisk, mix it together until well combined. Then add in the white sugar and continue whisking for a few more minutes.
  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. Using a spatula, combine them gently fold to create a soft cake batter. Do not over-mix! This is super important. As soon as you see no flour streaks, stop mixing.
• 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.
• Slice the cake and enjoy! This Sour Cream Coffee Cake is delicious and best served fresh or next day. You can keep it stored in the refrigerator in an airtight container for up to 5 days.