Line the base and the sides of a slice tin with baking paper, making sure you leave plenty hanging over the sides.
Place the biscuits in the bowl and crush for 5 seconds on speed 9.
Add the butter and heat for 3 minutes on 80 degrees speed 2.
Remove biscuit mixture from the bowl and pour into the lined tray. Push the mixture down with a metal spoon to create a flat surface. Place in the fridge for 30 minutes to set.
To make the filling, combine the gelatine in a jug with the boiling water and allow to cool slightly. In a clean bowl, add the lemon juice and condensed milk and mix for 45 seconds on speed 4. After 5 seconds, remove the measuring cup and carefully pour the gelatine mixture in. Remove the set slice base from the fridge and pour over the filling.
Put back into the fridge for 30 minutes.
In a clean Thermomix bowl, add the hot water and heat for 3 minutes on 90 degrees speed 2.
Add the jelly crystals and one extra teaspoon of gelatine and stir until they have dissolved. Pour jelly into another bowl and allow it to cool until it has reached room temperature.
Remove the slice from the fridge and carefully pour the jelly over the slice and place it back in the fridge to set for approximately 4 hours – overnight is best.
Remove the set slice from the fridge and cut into squares.