Heat the oven to 180C, 350F, Gas 4
Put the ricotta in a large bowl or electric mixer and mix in the egg yolks, followed by the flour, lemon zest and juice and finally the caster sugar. Mix until fully incorporated.
In a separate bowl mix the egg whites until really stiff and gently fold them into the ricotta mixture.
To make the base for the cake melt the butter in a small pan. Take off the heat and add in the crushed biscuits.
Spoon the mixture into a 24cm springfoam tin. Press down firmly with a spoon.
Pour the mixture over the cake and bake in the oven for 55 minutes.
Remove from tin when completely cool. Enjoy !