Slice tin: lightly grease with cooking spray then line a square tin (24cm x 5cm, 9" x 2") with 2 overlapping sheets of baking paper leaving a 1-2cm overhang on all 4 sides.
Chocolate fudge: add the chocolate pieces, sweetened condensed milk and butter to a heatproof jug. Microwave on high for 2 minutes then remove and stir vigorously for about 20 seconds. Return the jug to the microwave for 1 further minute (high heat). Then remove and vigorously stir for another 20 seconds.
Combine: add the chopped toblerone pieces to the combined fudge. Stir gently to combine.
Transfer: pour the fudge with combined toblerone pieces into the prepared slice tin. Pick it up and then gently tap the tin down again to the bench about 3 times (this removes the bubbles).
Set: place the fudge into the fridge to set for a minimum of 2 hours up to overnight.
To serve: remove the set fudge from the fridge. Grab the parchment paper and lift the fudge up and out of the tin. Discard the paper. Take a sharp knife and slice the large square into 4 smaller squares. Then slice each of those squares into 9 smaller pieces. Store the fudge in a sealed, air tight container until ready to serve.