Melt the butter in a saucepan.
Crush the digestive and amaretti biscuits in a plastic bag with a rolling pin.
Combine the melted butter, crushed biscuits, brown sugar, and ground almonds.
Press the mixture into a greased 24cm spring-form tin and chill for 30 minutes.
Mix the cream cheese, ricotta, lemon juice, zest, vanilla seeds, caster sugar, egg yolks, and double cream.
Beat the egg whites with a pinch of salt using an electric whisk until firm.
Fold the egg whites gently into the cheese mixture and pour into the cake tin.
Bake for 30 minutes at 160°C.
Increase the heat to 170°C and bake for another 30 minutes.
Turn off the oven and open the door.
Allow the cheesecake to cool completely inside the oven.
Dredge with icing sugar before serving.