Reserve 4-6 blackberries for decorating. In a small bowl crush blackberries with a fork or blend in a food processor. (You may add 1 tbsp of sugar to the blackberries if they are not sweet enough.)
In a large bowl whip double/whipping cream with icing sugar and vanilla until soft peaks form. Do not overbeat as you want softly whipped cream.
Divide whipping cream into equal parts. Take ⅓ of the mashed blackberries and fold gently into ½ of the whipped cream with a spatula.
Have 4 serving glasses ready for layering the blackberry fool. Start by dividing the remaining mashed blackberries among 4 glasses. Then top with blackberry cream and finish with vanilla flavoured whipped cream.Decorate each Blackberry Fool with one or two blackberries. Serve immediately or keep chilled until serving.