Preheat the oven to 350ºF and line an 8×8-inch aluminum cake pan with parchment paper and set aside. Set aside.
Prepare the streusel topping. Add the rolled oats, flour, brown sugar, butter, cinnamon, and vanilla extract to a mixing bowl. Use a spoon (or your hands work best) and cut the ingredients together until crumble forms. This may take 4-5 minutes. Set streusel topping aside for later.
Next, add the flour, baking powder, salt, and cinnamon into a bowl and toss. Set aside.
In a separate bowl, cream together the brown sugar, vanilla extract, 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 1 cup of milk to the batter. If the consistency is thick and creamy your batter is ready. If your batter is too thick like cookie dough, add 1 tablespoon of milk at a time until it reaches a thinner consistency.
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 half of the remaining non-cinnamon batter into the bottom of the cake pan and use a spatula to spread the batter out evenly. Pour the cinnamon batter into the cake pan and spread that layer evenly. Finish the cake with the rest of the non-cinnamon batter and smooth out the top layer.
Add the streusel topping to the top of the cake and gently press the streusel into the cake with your hands. Bake the cake for 30 minutes, cover it with a tin foil tent, and bake for another 30 minutes. Remove the cake from the oven if the middle of the cake is mostly baked (it will continue to bake for a few minutes after you remove it from the oven).
Let the cake cool for 15 minutes before serving.
