Melt the butter in a saucepan.
Meanwhile, crush the digestive and amaretti biscuits in a plastic bag, using a rolling pin.
Combine the butter, biscuits, brown sugar and ground almonds.
Press into the base of a greased, 24cm/9½in spring form loose-bottomed cake tin. Chill for 30 minutes.
Mix together the cream cheese, ricotta cheese, lemon juice and rind, seeds from the vanilla pod, caster sugar, egg yolks and double cream.
Add the salt to the egg whites and beat until firm, using an electric hand whisk.
Gently fold the egg whites into the cheese mixture and pour into the cake tin.
Bake for 30 minutes at 160C/300F/Gas2.
Increase the heat to 170C/325F/Gas3. For a further 30 minutes.
Turn off the oven and open the door.
Allow the cheesecake to cool in the oven.
Dredge with icing sugar to serve.