Place a heatproof bowl over a pan of simmering water, ensuring that the base does not touch the water. Break the dark chocolate into the bowl and allow to melt, stirring occasionally. Set the melted chocolate aside to cool slightly.
Halve and de-stone the avocados. Using a spoon, scoop the flesh into a food processor. Add the rest of the mousse ingredients and pulse for a few minutes. Scrape the sides down with a spatula then continue pulsing until it reaches a smooth consistency.
Pour in the cooled dark chocolate then pulse again to combine.
Divide the mixture into six small bowls then place in the fridge for 30 minutes.
For the Streusel topping:
Set the oven temperature to 150°C (Gas Mark 2). Line a baking tray with grease proof paper. Using the tips of your fingers rub together the flour, sugar and vegan spread until crumbly. Add the chopped nuts and seeds and combine thoroughly.
Place the mixture onto the baking tray and bake for 15 minutes or until golden brown.
Sprinkle on top of the vegan chocolate mousse and enjoy!