In a food processor, blitz the digestive biscuits to a fine crumb.
Melt the unsalted butter, add it in, and blitz again until combined.
Press into the base and sides of the tart tin so that its all evenly covered but also firm. Refrigerate for now.
In a large heatproof bowl over a pan of simmering water, add the 360g of Toblerone (chopped), double cream and butter and melt slowly, occasionally stirring.
Once smooth and melted, pour this onto the biscuit base and refrigerate for 2 hours.
Add the double cream and icing sugar into a bowl and whip.
Once whipped up, pipe onto the tart.
Decorate with some smaller pieces of Toblerone on the cream swirls, and then some chopped Toblerone pieces.
Notes
You can use a cake tin for this, or a larger tart tin, but these amount of ingredients perfectly fit a 23cm tart tin, thats about 1.5-2 inches deep.
I used one of the 360g (standard larger bars) of Toblerone for the filling, and then a 150g (standard smaller bar with the funny gaps in it) of Toblerone for the decoration. I use the actual triangles to go on the cream swirls, and the offcuts in the middle is what I chopped up to decorate with.
This dessert will last in the fridge for 3 days once set.
If your chocolate goes into small solid 'bits' in the tart, its likely cooled slightly too much so has seized. Next time you make it, don't let it cool slightly as much.