1.Spray a 20cm x 30cm (8-inch x 12-inch) rectangular pan with cooking oil; use baking paper to line the base and sides, leaving 5cm (2 inches) of paper hanging over the sides of the pan.
2.Put half the biscuits in a food processor; process until they have turned into fine crumbs. Chop the rest of the biscuits coarsely.
3.Put the butter and condensed milk in a small saucepan. Turn the stove on to low and stir with a wooden spoon until the mixture is smooth.
4.Put both lots of biscuits and the caramels in a medium bowl. Add the butter mixture; stir well. Put the slice mixture into the pan; press it down firmly. Cover the pan with plastic wrap and put it in the fridge for 30 minutes.
5.Meanwhile, make the chocolate topping: put the ingredients in a small heatproof bowl; place the bowl over a small saucepan of simmering water on the stove on low heat. Stir with a metal spoon until the topping mixture is smooth.
6.Take the pan out of the fridge; spread the chocolate topping over the slice with a palette knife. Put it back in the fridge for about 30 minutes or until the topping is firm.
You can keep the slice in an airtight container, in the fridge, for up to four days. Tim Tams are chocolate-filled chocolate biscuits covered in chocolate. Jersey caramels are confectionery made from sugar, glucose, condensed milk, flour, oil and gelatine.