Preheat oven to 180°C. Grease a 20cm x 30cm lamington tin and line both sides with baking paper, allowing paper to overhang the sides. Put butter, chocolate and marshmallows in a saucepan and stir over a low heat until marshmallows are almost melted.
Pour mixture into a large bowl and whisk until marshmallows have dissolved. Allow to cool slightly. Whisk in eggs and sugar. Sift flour and cocoa over mixture and whisk in until combined. Stir in pecans.
Pour into prepared tin. Bake for 40-45 minutes or until a skewer inserted in the centre comes out clean. Allow to cool in tin. Use baking paper to help lift from tin, then put on a chopping board and trim ends. Cut into 9 squares.
To make sauce, whilst cake is baking, put all sauce ingredients in a saucepan and stir over a medium heat until sugar has dissolved. Bring to a simmer. Simmer gently for 10-12 minutes or until mixture turns into a thick dark caramel sauce. Cool slightly.
Serve squares of fudge pudding topped with a scoop of ice-cream and drizzled with sauce.