Preheat the oven to 180ºC.
Melt butter, chopped dark and milk chocolates over a double boiler until smooth and glossy.
In a mixer bowl, add in white sugar and brown sugar, then whisk in 1 egg at a time. As you add in the 3rd egg, add 1 tsp vanilla extract, and whisk until creamy. Tip: Do not overmix to avoid making brownies with a cakey texture.
Pour in the chocolate ganache and mix until just combined.
Sift in the dry ingredients - plain flour, cocoa powder, and salt. Fold the batter until just combined.
Pour the batter into a 7” x 7” square pan lined with parchment paper.
Bake in the preheated oven at 180ºC for 25-30 minutes. Insert a toothpick or a baking needle into the centre. For a fudgy texture, it should come out with a few moist crumbs clinging to it.
When the brownies are done, remove them from the cake pan and let cool completely.
Meanwhile, melt some extra dark chocolate and pour into a piping bag. Pipe ghostly faces on the marshmallows, and prepare the icing glaze by mixing powdered sugar and milk until no lumps. Adjust the consistency by adding in more or less liquid.
When the brownies are cooled, divide into 9 pieces. Use a spoon to spread the icing glaze on each brownie base, then place a marshmallow ghost on top. Tip: Do this step quickly to prevent the icing glaze from drying out.