Prepare a 23 cm springform baking pan/tin by greasing and covering the base with non-stick baking paper.
Preheat the oven to 150°C for a conventional oven, 135°C for a fan-forced oven, or 300°F (gas mark 2).
Make the cheesecake base by crushing the biscuits (easiest way is in a food processor) until they look like breadcrumbs, add the melted butter and stir until combined. Press into the base of the prepared tin and place in the freezer for about 30 mins.
Beat the cream cheese until light and fluffy in a mixing bowl, and gradually add the can of sweetened condensed milk, ensuring that it is beaten well and all mixed together. Beat well after each addition of the egg, then add the vanilla and beat once more.
Pour the cheese mixture onto the base and bake in the oven for about 1 hour.
Turn the oven off, open the door and leave in the oven to cool for about 1 hour. Remove from the oven and allow to completely cool at room temperature. When cool, refrigerate for a few hours until firm.