Preheat oven to 180oC.
On a stove top, melt the butter in a 12 inch cast iron skillet over medium heat. Stir often until completely melted. Turn heat to low and add the caster sugar and brown sugar, stirring well. Keep stirring until smooth.
Chill skillet in the fridge for 10-15 minutes. Remove from the fridge and add eggs and vanilla. Stir well.
Stir in the flour, Bi-Carb Soda and salt. Mix in the chocolate and coconut and stir to combine.
Bake in the pre-heated oven for 25 minutes or until the edges are lightly golden brown. It will still be gooey in the centre.