Melt the butter and brown sugar in a pan over a medium heat until bubbling. Add the Crème de Cassis and flambé. Toss in the blackberries and stir gently for one minute. Remove from heat and leave to cool.
Whisk the mascarpone, double cream, icing sugar, vanilla seeds and vanilla extract in a bowl. When smooth, stir in the lime zest and the orange juice.
Place the biscuits in a plastic bag and beat gently with a wooden spoon until they resemble rough breadcrumbs (If you cannot find almond and pistachio cookies you could substitute Amaretti biscuits).
To assemble the Cosmo place some crushed biscuits at the bottom of a martini glass. Put a large dollop of the mascarpone mixture on top and place a layer of blackberries on top of that, followed by another layer of mascarpone cream. Repeat layers again and garnish with some blackberry juices.