Preheat the oven to 180°C (160°C fan forced). Grease a 20cm square shallow cake pan and line the base and sides with baking paper.
Open the brownie box, remove enclosed frosting sachet and reserve for another use. Mix the brownie mix, butter and eggs together in a bowl until well blended. Spread the mixture evenly in the prepared cake pan and bake for 20-25 minutes or until a round-bladed knife inserted into the centre comes out almost clean. Remove from the oven. Transfer to a wire rack and leave to cool completely in the pan; about 30 minutes.
Lift the brownie from the pan using the baking paper. Cut the brownie into 16 squares.
Place the frosting in a small bowl and use the orange food colouring paste or gel to colour the frosting bright orange.
Spread equal amounts of frosting onto each brownie, using the tip of a small palette knife to swirl the frosting into a pumpkin shape.
Pipe a Halloween pumpkin mouth, nose and eyes on each brownie with some of the black writing icing. Use the green writing icing to pipe a green stalk on each brownie. Serve immediately or keep in a cool place for up to 2-3 days.