Preheat the oven to 180C / 350F / 160C fan forced. Line an 8x8 inch square baking tin with baking paper.
On the VERY LOWEST heat, melt the butter and chocolate together in a saucepan, stirring. Remove from heat.
With a whisk, whisk together both sugars, eggs plus the melted chocolate and butter.
Sift over the flour, cocoa and salt and use a spatula to mix through until just combined.
Bake for 20 minutes
Place the marshmallows in a microwave safe bowl, then microwave on high for around 50 seconds.
Spray the edges of the baking paper that are sticking up around the brownie with spray oil. Also spray a spatula, then give the marshmallow a stir and tip it out onto the brownie.
Wet your hands lightly and press down over the top of the marshmallow to even it out.
Change the oven to the overhead grill / broiler on high, then toast the marshmallows for about 2 minutes. Keep an eye on them so they don't burn (notes).
FOR THE EASY CHOCOLATE ICING
Once the brownie has cooled, transfer them to a tray or rack lined with baking paper.
Sift the icing sugar and cocoa into a bowl. Add 1 tablespoon of milk and mix until you have a chocolate glaze. If it seems to thick add a little more milk a couple of drops at a time. It can get runny too quickly.
Drizzle all over the top of the cooled brownies and allow to set before cutting (notes).
I use a standard Australian 20ml tablespoon (= 4 teaspoons worldwide)
Burnt marshmallow: Uhoh! You left it under the overhead grill / broiler too long and now the marshmallow is burnt. No problem. You can use a knife to gently peel away the burnt layer and re-grill / broil the top. You can also use a kitchen blowtorch for this step if you prefer.
The chocolate drizzle: Add the milk slowly. It needs to be a consistency that will run but also will set and not just run straight off the marshmallow. Add milk and stir until it's just running off the spoon.
Cutting marshmallows: This can become a sticky situation. My favourite method is to rub a large chefs knife lightly with oil, then dip it in icing (powdered) sugar. Re-dip in the sugar before each slice.