Place the oven rack in the lower third of the oven and preheat it to 250°F (120 °C). Line the baking paper.
Place egg whites and salt in a large bowl and mix with an electric mixer at medium to high speed until foaming. Add tartar cream and continue mixing until a soft peak is formed. While mixing, add 1 tablespoon of powdered sugar at a time. Continue beating until a hard peak is formed.
Add green food colouring and allow 15 seconds of whipping until combined then put the meringue in a clean piping bag with a large star-shaped tip. Pipe the tree (pipe a large star and then two smaller stars on top to create a tree) to a baking sheet lined with parchment paper. If used, top with sprinkles to make it look like a decorated Christmas tree.
Bake for about an hour until the meringue becomes firm and feels light and dry when picked up.
Open the oven for a few minutes, then turn it off and close the door. Leave in the oven for about 2 hours until the meringue is completely dry.
Serve them immediately; or You can freeze these cookies for up to 1 month if stored in an airtight container.