Place all the ingredients for the base into a food processor and pulse until combined. Don't over process it or it will turn into a paste, you still want some crunch in it and the base to hold together. Transfer it into a loose bottom tin and flatten it out evenly. I prefer loose bottom tins because it is so much easier to get cake out of them, especially no-bake cakes. The base will seem crumbly and rather sticky but don't worry, once it's frozen it will hold together very nicely. Transfer the form into the freezer and proceed with the cream.

Half the avocados, remove the stones and scoop the flesh into the bowl of a food processor. Make sure the avocados are ripe. If you are unsure about how ripe they are you can look at this post. Remove the pits from the dates and add them with the rest of the cream ingredients into the bowl. Process for several minutes until perfectly blended. Depending on your avocados and dates you might need a few more spoons of water. The consistency should be silky smooth.

Take the cake from out of the freezer and tip the cream onto the base. Spread it evenly with a spatula and tap the form against your kitchen counter a few times to release possible bubbles that are caught in the cream. Return it to the freezer and let it sit for about 2 hours.

Once frozen, spread a layer of berries on top. There is an easy way to get a neatly looking cake without spending an hour on arranging the berries. Simply lay a neatly placed ring of berries at the edge of the form and tip the rest of the berries into the center. Spread them gently with your fingertips and you are done. This will take little time and look like you arranged each single blackberry.

Return the avocado chocolate mousse cake to the freezer and take it out about half an hour before you want to eat it. Keep in mind this is a mousse cake. It will be very soft, creamy and rich so if you do want perfect slices you will have to keep it a bit more on the frozen side.