Preheat oven to 180ºC fan forced, and place rack into middle of oven.
Grease and line tin (21cm square tin, or 26cm x 16cm oblong tin).
Place dark chocolate into TM bowl and mill 10 sec/speed 9.
Add butter and melt together 2 min/50ºC /speed 3.
Add sugar and mix 5 sec/speed 4.
Add vanilla bean paste and eggs and mix 10 sec/speed 4.
Add flour and salt and mix 1 min/speed 4.
Add prunes and chocolate buttons and mix 20 sec/Reverse/speed 1.
Pour mixture into tin and spread to the corners.
Bake for 25 minutes then check it is cooked by touching gently with your finger in the middle - it should still be soft, but if it is very soft bake for a further 2-3 minutes.
Remove from oven and allow to rest in the baking tin until cool. Dust with cacao powder before cutting into squares.