Preheat oven to 180°C (160°C for fan assisted ovens) Gas Mark 4. Line the bottom and sides of an 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 a cooling rack to cool completely; about 1 hour. Freeze in tin for 30 minutes.
Remove brownies from the tin using the foil to lift and peel away. Use a sharp knife to cut the brownies into 5 rows by 3 rows, making 15 brownie squares (about 5cm x 5cm each).
Spoon the icing into a medium bowl. Stir the food colouring into a bowl to your desired shade of red.
Ice the top of the brownies with the red icing and sprinkle with 100's and 1000's.
Place 6 green sugar-coated chocolates in a triangle on top of each iced brownie to form a tree. Place 1 yellow mini chocolate on top of the tree shape to create a star. Place one piece of liquorice at the bottom of the triangle for the tree trunk.
Store covered at room temperature.