Preheat oven to 160°C. Line a square 25cm slice tin or baking dish with baking paper.
Melt first measure of butter in a large bowl. Stir in base mix, until combined. Press base mixture firmly into prepared slice tin, using the back of a spoon to create an even layer. Bake on middle oven rack for 20-25 minutes, or until lightly golden.
In a small pot on medium heat, bring the butter and brown sugar to a simmer and cook for 3-4 minutes. Add in condensed milk and cook for a further 2-3 minutes, stirring continuously, until lightly golden. Removed from heat and set aside.
When base is cooked, remove from oven. Pour over caramel and spread out into an even layer. Return to middle oven rack for a further 15 minutes, until caramel is bubbly and golden.
Break up chocolate and place on top of hot caramel so that it melts, use a spoon to spread out evenly. Allow to cool and then refridgerate.
Slice cut into squares.