Line a 8x8 inch (or equivalent) baking tin/pan with baking paper and set aside. Preheat the oven to 345F/175C.
Blend the avocado and banana together in a small food processor. Or mash well with a fork.
Melt half of the chocolate (3.5 oz/100 gr) in the microwave in 30-second bursts. Let cool slightly.
In a large bowl, whisk the eggs into the avocado and banana mixture with a whisk or fork. Then whisk in the maple syrup and cocoa.
Slowly pour and whisk in the melted chocolate. Then stir in the flour, baking soda and salt with a spatula until just combined.
Smooth the batter out into the prepared pan. Bake for 17 minutes or a little longer (a minute at a time) until the top of the brownies are just set and a skewer poked into the centre comes out a bit sticky (but not with runny raw batter on).
Let the cake cool for a while on a wire rack. Then melt the rest of the chocolate with about ½ teaspoon oil. Pour over the brownies. Leave to set for a while, then slice into 16 brownies.
How to blend the avocado and banana: I usually do this in the mini food processor that came with my hand-held blender. If you don't have anything similar to this it's fine to just mash them together with a fork. The riper the better if you're going to do it this way, of course!Cooking time: I usually bake these brownies for anywhere between 17 and 19 minutes. After 17 minutes, poke a skewer into the centre. You want it to come out still a bit sticky (but not with runny raw batter on). It's better to undercook slightly than to overcook these. A couple of times I've left mine in the oven a minute or two too long and the brownies have been slightly dryer around the edges. But they still taste moist and fudgy, especially with the chocolate on top! Topping: I usually drizzle melted dark chocolate over the top of my brownies before slicing. If you prefer though you can just toss in a handful of dark chocolate chips instead! How to store: You can keep your brownies in an airtight container at room temperature for a few days. Alternatively, keep them in the fridge in a sealed container - they'll taste very fudgy!