Place the biscuits/graham crackers into a large bag or bowl and crush them into crumbs using the end of a rolling pin. Add the melted butter, brown sugar, and salt and mix until the crumbs absorb the butter.
Place the blueberries, blackberries, lemon juice, water and sugar in a pan and heat gently until the berries start to break down (about 5 minutes).
Stir in the cornflour mixture and heat through, continuously stirring for another 30 seconds until the berry mixture has thickened. Turn off the heat and allow to cool.
Pour the cream into a large bowl and whisk with an electric whisk until thick.
Add the cream cheese, vanilla and confectioner’s sugar and whisk again until combined. The mixture should be thick enough to hold its shape.
Divide the cookie crumbs between 6 mason jars (mine are 200ml/7oz jars) and press down with the back of a spoon.
Spoon or pipe the cream cheese mixture on top, then spoon over the cooled berry mixture.
Sprinkle with lemon zest and serve – or place a lid on and refrigerate until ready to serve.