• Preheat the oven to 350℉/176℃. For easy removal of the cake, place parchment paper on the bottom and 2 sides of the pan. Gently grease the other two sides and then set aside.

For the streusel

• In a medium sized bowl, add flour, brown sugar, white sugar, cinnamon, and softened butter. Mix this with a fork or using your hands until combined. Sprinkle another tbsp of flour on top and crumble the streusel into bigger and smaller pieces. You may need 1 more tbsp of flour depending on how soft your butter is. Place in the fridge when done to chill.
  3/4 cups all-purpose flour + 2 tbsp, 1/4 cup white sugar, 2 tbsp dark brown sugar, 1/2 tsp cinnamon, 5 tbsp butter

For the cake

• In a medium sized bowl, add in the dry ingredients. Stir to combine and set aside.
  2 cups all-purpose flour, 1 tsp baking soda, 1 tsp cinnamon, 1 tbsp cornstarch, 1/4 tsp salt
• In a large bowl, add softened butter, brown sugar, and white sugar. Beat on medium speed for 2-3 minutes until it’s light and fluffy. Scrap the sides of the bowl as needed.
  1/2 cup unsalted butter, 1/2 cup dark brown sugar, 1/2 cup white sugar
• Add the eggs and vanilla into the bowl and mix on medium speed until well combined. Make sure to scrap the sides of the bowl.
  2 large eggs, 1 tbsp vanilla
• Add in the mashed bananas and continue to mix on medium speed for another minute, scraping the sides of the bowls when needed.
  1 1/4 cups bananas
• Slowly add the dry ingredients in three additions, starting and ending with the dry ingredients. Alternate between adding in the dry ingredients and the sour cream. I used an electric mixer to add in the ingredients until the last bit of flour, I mixed that in with a spatula. Mix until just combined between each addition.
  1/2 cup full-fat sour cream
• Add the batter into the prepared pan and spread evenly. Sprinkle the streusel on top of the batter.
• Bake the cake for 45-60 minutes. The cake will be ready when a toothpick inserted comes out dry. Mine took 55 minutes.