Mix the biscuits, butter, lemon zest and salt. Transfer the mixture to the prepared tin, press flat, press down firmly, chill for approx. 30 mins. Spread the jam over the top.
Combine the cream cheese with all the other ingredients up to and including the salt, mix well using the whisk on a hand mixer. Stir in the blackberries, spread over the base.
Approx. 60 mins. in the lower half of an oven preheated to 170°C. Remove from the oven, leave to cool in the tin, cut into squares. Mix the blackberries, lemon juice and icing sugar, serve with the cake.