Prepare the barbecue for indirect cooking over medium-low heat (170°C to 180°C). If using a Weber Q barbecue, set up your barbecue with a convection tray and a trivet.
Place the butter and sugar in a medium sized bowl and beat until pale and creamy. Stir in the flour and mix until soft a dough forms. Press the dough into the bottom of a Weber frying pan, or a shallow baking dish.
Bake the base over indirect medium-low heat, close the lid and cook for 10 minutes or until lightly browned.
While the base is baking, mix the condensed milk, coconut, cranberries, cherries, pistachios and in a medium sized bowl. Once the base is cooked, gently pour and spread the condensed milk mixture over the base. Close the lid and continue to cook the slice for a further 10 minutes.
After 10 minutes, carefully push the marshmallows into the slice. Continue to cook the slice for a further 5 minutes.
Remove the slice from the barbecue and set aside to cool.
Melt the baking chocolate in a microwave. Spread the melted chocolate over the slice. Leave the slice to set overnight.
Once the slice has set, cut into squares and store in the fridge.