Preheat oven to 165°C.
Using electric beaters, cream the butter and sugar until soft.
Add the flour and gently mix until combined.
Pat firmly into a paper-lined 20 x 30cm baking pan and bake for 20 minutes or until the top turns a light golden colour.
Remove from oven and top immediately with the broken pieces of Toberlone, smattering evenly across the slide.
Leave for 5 minutes, then using a spatula or flat spoon, spread the softened chocolate evenly over the slice.
Refrigerate to cool and set for at least one hour.
Using a hot knife, cut into scrumptious slice.