Position a rack in the middle of the oven. Preheat oven to 350 degrees F (176 C) and butter an 8 inch round baking pan (or 8x8 rectangular pan // adjust pan size if altering batch size) with vegan butter. Toss with some white rice flour (or other GF flour) to coat the edges and remove excess. Set aside.
Add avocado and banana to a large mixing bowl and mash.
Add honey (or maple syrup or agave), vanilla, coconut oil and stir. Measure out almond milk and toss in the vinegar. Let set for a minute or two then add to the mixture and whisk.
Next add baking soda and powder and mix vigorously to combine.
Add cocoa powder, salt, gluten-free flour blend, and coconut flour to a sifter (reserving oat flour since it's more coarse) and sift over wet ingredients. Whisk vigorously to combine.
Last, add oat flour and stir once more. Taste a bit to see if it needs more honey. Then pour into the prepared baking pan and smooth the top with a rubber spatula.
Bake for 40-48 minutes or until a toothpick inserted into the center comes out clean. Cracks on the top are fine and actually good. The edges should be pulling away from the sides just slightly.
Remove from oven and let cool in the pan for 1 hour. Then, loosen the sides with a knife and place a plate on top and gently flip over. Repeat with another serving plate so it's right-side up. Dust with cocoa powder (optional) and serve immediately. Refrigerate if keeping longer than a few days. Freeze for even longer storage. Frosting is really not necessary. It's very rich and fudgy on its own.
*Nutrition is a rough estimate. *When you pull the cake out of the oven, it will look like it's not done in the center. That's fine, it is. Plus, it's egg-free so it's fine if it's a little fudgy in the middle. However, cook for 5 minutes more if you're concerned. Cooling in the pan for 1-2 hours will finish "cooking" it and will let it set.