In a large mixing bowl, beat egg whites on high speed until it reaches soft peaks. Sprinkle in the cream of tartar while mixing.
In a small bowl, stir together sugar and cornstarch. With the mixer still on high speed, sprinkle the sugar mixture into the egg whites one spoonful at a time. Beat until meringue reaches glossy stiff peaks.
Portion out a few spoonfuls of meringue into small bowls and mix in green gel food colouring. Transfer all colours of meringue into separate piping bags fitted with piping tips of your choice.
Preheat oven to 250°F. With a pencil, trace a 6” circle on the back of a sheet of parchment paper and place it on a baking sheet.
Following the circle on the parchment paper, pipe swirls of white meringue in a circle. Repeat with a second layer on top of the first ring of meringue.
Using the green coloured meringue, pipe trees on top of the wreath. Decorate with small sprinkles. Any larger sprinkles can be added after baking with a dab of melted chocolate, if desired, otherwise they may melt during baking.
Bake for 1.5 hours or until the outside walls are dry and hard and the bottom peels cleanly off the parchment paper.
Turn off the oven and leave the pavlova inside the oven for about 30 minutes to cool. Remove from the oven and let it finish cooling completely on a wire rack. Serve as is or with whipped cream and fresh fruits on the side.