Crumble First.
Step 1- Mix flour with flour, then stir in sugar and oats, should be a crumbley consistency (set aside), do not add the clusters yet.
Cake
Step 1- Pre-heat oven to 350F
Step 2 - Mix dry ingredients together
Step 3- Add eggs, bananas, and oil. If the mixture seems a little too dry, add a touch more oil. I would suggest adding the oil as you stir. The final batter should be fluffy and loose but not too loose so that it's like a liquid.
Step 4- In a 8x8 dish, place parchment paper and spray some cooking spray on it. This will make removing the cake a breeze.
Step 5- Pour half of the batter into the dish. Spread about half of your crumble topping on top of the batter.
Step 6- Then pour the rest of the batter over it. Place in the oven.
Step 7- Bake in the oven for about 45 to 50 minutes (at about the 30 minute mark, add the rest of the crumble topping to the top of the cake along with the clusters), do the toothpick check, if no batter comes out with the toothpick it's ready.
I like to pull the cake out when it's still slightly loose and I allow it to cook, because it makes for a really yummy moist cake. It's an inexact science.