Begin by preheating your oven to 350°F (175C/155C Fan) and lining an 8" square baking tray with parchment paper.
In a large mixing bowl, whisk together the wet ingredients - the eggs, oil, water and vanilla.
In a separate bowl, stir together all dry ingredients - the sugar, powdered sugar, flour, cocoa powder and salt. Add this to the bowl of wet ingredients and stir with a spatula to combine into a thick brownie batter. Try to avoid over mixing but ensure no lumps of unmixed dry ingredients are left.
Pour this batter into the prepared baking tin and gently smooth into an even layer, nudging the mixture into the edges and corners with a spatula.
Bake in the centre of the pre-heated oven for about 30 minutes. For fudgey chocolate brownies, check for a toothpick inserted in the centre to come out with a few bits on it.
Remove from the oven temporarily in order to sprinkle with coconut and mini marshmallows then place back in the oven for about 3 minutes until the marshmallows begin to look a bit swollen.
Remove the brownies from the oven again and allow to cool in the tray while to prepare the chocolate topping.
Bring a small amount of water to a simmer in a saucepan and turn down to a low heat. Meanwhile, place the chocolate in a small bowl, that will sit snuggly over the saucepan (like a DIY double boiler).
Gently melt the chocolate in this DIY double boiler. Then remove from the heat and add the butter, stirring until melted into a glossy thick chocolate spread.
Drizzle or spread this over the marshmallows and allow to cool for at least two hours before removing from the pan and slicing. (see note)