In a bowl, whisk together the flour, baking powder, baking soda, salt and chia seeds.
In a larger bowl, whisk together the sugar, oil and applesauce.
Add the bananas and vanilla and mix well.
Add the flour mixture and mix until everything's moistened and well-mixed.
Oil and flour a cake pan. I used a square pan that's 9 X 9 inches but an 8 X 8 inch pan would be perfect too. Or use an 8- or 9-inch round cake pan.
Pour the batter into the pan.
Prepare the crumb topping by mixing all the ingredients-- the flour, sugar and oil-- together until everything's moistened.
Sprinkle the crumb topping evenly over the top of the cake batter.
Preheat the oven to 350 degrees Fahrenheit. Place the pan on the center rack and bake 30-35 minutes or until a toothpick inserted in the center comes out clean.
Cool on a rack, then cut into squares and eat.