I greased my pan with a coconut baking spray. You can use anything you wish, ranging from solid fats (butter, coconut oil, etc) to liquid fats (olive oil, avocado oil, etc.) so long as you get good coverage.
I am calling the Earth Balance Baking Stick butter in the instructions because the full name is too long, and saying "stir in the melted baking stick" is just weird. I trust you will forgive me. The same is true with other ingredients. Flour, Butter, Sugar - they are all substituted. I was more specific for the eggs, of course.
Be sure to use a tablespoon of ground flax. A tablespoon of unground flax will yield more once ground, so measure after you grind. I use a spice-dedicated coffee grinder for all my grinding needs.
When making the flax egg, it needs some time to set up. It should thicken before you use it. If it is too watery, add a little more ground flax and give it more time, a minute or two should be all you need.
Any bananas will do for this recipe, but think about it like banana bread - the riper the banana, the moister and creamier it will be. It will also combine better.
This dough is really sticky. Once I got in the first half, I spread it around with a silicon spatula as best I could, but when I added the streusel, I was able to use it to smooth it out. I did the same on the top.
Note: I do not suggest making brown sugar with Lakanto and molasses for this cake. I have had the best results with conventional brown sugar.
Adapted from Great American Home Baking