Preheat the oven to 350℉ and line an 8x8-inch cake pan with parchment paper. Set aside.

Prepare the streusel topping. Add the rolled oats, flour, brown sugar, butter, cinnamon, and vanilla extract to a mixing bowl. Use a fork (or your hands work best) and cut the ingredients together until a crumble forms. This may take a few minutes. Set streusel topping aside.

Next, add the flour, baking powder, salt, cinnamon, and carrot into a bowl and toss to combine. Set aside.

In a separate bowl, cream together the brown sugar, vanilla extract, maple syrup, and butter until light and fluffy with an electric mixer.

Drizzle the avocado oil into the creamed mixture and mix together until combined. Add the eggs and mix to combine. Add ½ cup of milk to the batter.

Add the dry ingredients to the wet and mix to combine. Remove one cup of the batter and transfer it to a bowl. Add 2 teaspoons of cinnamon to the 1 cup of batter and stir to combine. The batter with cinnamon should be a darker cinnamon color.

Next, pour the large batch of the batter into the cake pan. Then, pour the cinnamon batter on top. Using a knife, swirl the cinnamon batter into the rest of the batter.

Add the streusel topping to the top of the cake and gently press the streusel into the cake with your hands. Bake for 20 minutes. If the streusel topping is getting dark, you can cover it with tin foil. If not, continue baking for another 20 minutes.

Let the cake cool for 15 minutes before serving.