Preheat oven to 180°C (160°C for fan assisted ovens) Gas Mark 4. Line the bottom and sides of a 11x7-inch rectangular tin with foil and well-grease. Make and bake brownie mix as directed on box using water, oil and eggs. Transfer to cooling rack to cool completely; about 1 hour. Freeze in pan for 30 minutes.
Remove brownies from the tin using foil to lift and peel away. Use sharp knife to cut brownies into 5 rows by 3 rows, making 15 brownie squares (about 5cm x 5cm each).
Spoon icing into medium bowl. Stir food colouring into bowl to your desired shade of green.
Ice the top of the brownies using the green icing and sprinkle with 100's and 1000's.
Place 3 red sugar-coated chocolates in a triangle on top. Cut 2 mini marshmallows in half crosswise. Place 3 marshmallow halves for the hat’s brim and one on top of triangle for pom-pom. Repeat for all brownie squares.
Store covered at room temperature.