Preheat oven to 180ºC bake and line a 20cm square cake tin with baking paper.
Process or crush biscuits into crumbs. Combine with melted butter and press into the bottom of the prepared tin to form a thin layer.
Place brownie mix, butter, eggs and water in a medium mixing bowl. Mix with a wooden spoon until well combined and smooth. Spread brownie mixture over the biscuit layer and smooth the top.
Bake for 35 minutes, until top is no longer wobbly and a skewer inserted into the centre comes out with a few moist crumbs. Allow to cool completely in the tin.
Place frosting mix, butter and milk in a medium mixing bowl. Using an electric mixer, beat on low speed for 30 seconds to combine. Increase speed to high and mix for 3 minutes, scraping down the sides of the bowl occasionally. Spread over the top of the cooled brownie.
Cut brownie into 25 squares (approximately 4cm) and gently press a marshmallow on top of each square. Optional step: use a blowtorch to carefully toast the top of each marshmallow.
Break chocolate into squares and place in a microwave-safe bowl. Microwave on medium-low in 30 second bursts, stirring in between, until melted and smooth. Stir in vegetable oil. Spoon chocolate all over the top of the marshmallows, letting it drip down the sides of the brownie to achieve a “spooky ghost” look. Refrigerate for 15 minutes, until chocolate has set. Using the icing pen to draw eyes and a mouth on the ghosts.