For the caramel, pour the condensed milk into a glass bowl and place over a saucepan of simmering water, making sure that the bottom of the bowl does not touch the water. Whisk every 20 minutes for around 2.5-3 hours, or until the mixture turns a caramel brown color.
Leave to cool to room temperature, then finely dice the apple and stir through the caramel.
Stir together the biscuit crumbs and melted butter. Tip into an 8” loose-bottomed round tin and use a cup measurement to pack into the bottom and sides of the tin. Place into the fridge until needed.
In a medium bowl, whisk the cream to medium peaks. Set aside.
In a large bowl, whisk together the caster sugar, sour cream, icing sugar, lemon juice and vanilla extract. Gently fold through the whipped cream.
Tip the filling into the prepared biscuit base and leave to set for 2-3 hours in the fridge.
Top with the caramel apple sauce, slice and serve.