Place the biscuits and the butter in a food processor and blitz to a coarse crumble. Press the mixture into the base and sides of your tart tin. Chill for about an hour.
Zest and juice the lemons into a saucepan. Add the sugar and the butter (cut into small dice). Cook over a medium heat to melt the butter and then reduce the heat and whisk in the egg yolks, one at a time. Cook until the mixture thickens and resembles custard. Remove from the heat and allow to cool.
Once cool, spoon the mixture into the tart case and smooth down the surface to create an even layer. Place in the fridge and leave to set for at least two hours.
Arrange the blackberries on top and dust with icing sugar to serve.
«Add a glazed finish to the tart by warming apricot jam and brushing over the blackberries and the curd.»