Line a 18cm x 28cm rectangular slice tin with baking paper and set aside.
Prepare jelly according to the packet instructions and set aside on the bench to cool. Do not place the jelly in the fridge as you don't want it to set just yet.
Place the butter into the Thermomix bowl and melt for 2 minutes, 80 degrees, Speed 2. Add the biscuits/cookies and mix for 8 seconds, Speed 8 (you may need to scrape down the sides of the bowl partway through). Firmly press the mixture into the bottom of the prepared tin and then place it into the fridge while you prepare the filling.
Place the sweetened condensed milk and lemon juice into a clean Thermomix bowl. Add the dissolved gelatine mixture (see tips below) and mix for 15 seconds, Speed 4.
Pour the mixture over the prepared base and place into the fridge until set (approximately 1 hour).
Gently pour the cooled jelly over the top of the slice and place into the fridge for 5 hours or until set.
Cut into slices and store in an airtight container in the fridge for up to 3 days.