• Preheat the oven to 350°F/175°C and prep a 9x9 baking pan. Combine brown sugar and cinnamon in a small bowl and set aside. This will be used in 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 softened butter. Mix this until combined and crumble the streusel into smaller and larger pieces. Set aside in the fridge when done.
  1 cup all-purpose flour, 1/4 cup + 1 tbsp dark brown sugar, 1/4 cup + 1 tbsp white sugar, 6 tbsp butter, 1/2 tsp cinnamon

For the cake

• In a medium-sized bowl, add in the dry ingredients. Stir together and set aside to use later.
  2 cups all-purpose flour, 1 1/2 tsp baking powder, 1/2 tsp baking soda, 1 tbsp cornstarch, 1/4 tsp salt
• In a large bowl, add oil, butter, and brown sugar. Whisk until well combined (2-3 minutes). Then add in the eggs, sour cream, and vanilla. Continue whisking until nice and smooth, and the mixture is lighter in colour and has a bit of bubbles.
  1/4 cup butter, 1/3 cup oil, 3/4 cups dark brown sugar, 2 eggs, 3/4 cups full-fat sour cream, 1 tbsp vanilla
• Add the dry ingredients into the wet and gently mix together with a spatula to create a soft cake batter. Don't over-mix!
• In a small bowl, add the blueberries. Take 1 tbsp of flour and sprinkle it in top. Mix and make sure the blueberries are all coated in it. Tossing the blueberries in flour helps prevent them from sinking.
  1 1/2 cups fresh blueberries
• Add half of the batter into your prepared pan and spread evenly across the pan, making sure the entire bottom of the pan is covered. Then sprinkle all of the cinnamon sugar topping, about 4-5 tbsp of the streusel, and 1/2 of the blueberries. Add the remaining half of your batter as evenly as possible, it doesn't have to be perfect. Then sprinkle the rest of your blueberries and streusel on top. I recommend dolloping the batter all around and gently spreading it this way. This helps to evenly distribute the batter without disturbing the filling layer.
• Bake the cake for 45-60 minutes, or until a toothpick inserted comes out dry. Mine took 55 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.