Bring a medium saucepan of water to a simmer. Preheat oven to 90°C (194°F). Line a baking tray with baking paper. Set aside.
In a metal bowl, combine the egg whites, sugar and cream of tartar (lemon juice/white vinegar). Place the bowl over the simmering water and stir constantly until the sugar has dissolved. This usually takes about 3-4 minutes. You can test to see if the sugar has dissolved by rubbing a bit of the egg whites/sugar between your fingers. It should feel smooth, not grainy.
Remove bowl from the heat and beat with a hand/stand mixer with a whisk attachment on low/medium speed for a couple of minutes. Increase the speed to high and beat until stiff peaks form.
Add the vanilla extract and mix until well combined. (In total the whole process will take about 10 minutes.) If using food coloring, add together with the vanilla extract. Makes the meringues more fun for the kids. =)
The meringue should be stiff enough, that if you flip the bowl upside down, nothing will fall out.
Transfer to a large piping bag fitted with a large star tip. Pipe 4 cm (2 1/2") diameter meringues onto a baking tray lined baking with baking paper. Depending on how big you make the meringues, it should make about 20-24 meringues.
Bake at 90°C (194°F) degrees for 2 1/2 - 3 hours (again, depending on size of meringues). You can break one after about 2 - 2 1/2 hours to see if it dried through. (I bake them for about 1 1/2 hours, and they are still a bit sticky, just the way I like them.)
If after 3 hours they are still wet or sticky, add another 15 minutes, and check again. Keep adding 15 minutes until completely dried through.
Allow to cool completely before adding any buttercream or any kind of cream. This this take about 15-20 minutes. You can stick two cookies together with some butter cream, or eat them just like that. They simply melt in your mouth.
Enjoy!