If you are making the optional crumble topping, preheat the oven to 190C/170C Fan/Gas 5.
For the mousse, melt the chocolate in a heatproof bowl set over a saucepan of gently simmering water, or in the microwave. Take off the heat and stir until smooth. Leave to cool for a few minutes.
Put the avocado into a blender and blitz until smooth. Add the chocolate in a slow steady stream with the motor running and blend until thick and creamy. Divide among four small glass tumblers and place in the fridge.
To make the crumble topping (optional), mix the melted spread with the flour and sugar to make chunky clumps. Scatter over a baking tray and bake for 10–15 minutes, or until golden-brown and crisp. Leave the crumbs to cool for a few minutes, then sprinkle over the chocolate mousse.