Use this black meringue to form the heads and legs of the scary Spider Cakes for Halloween.
Fill a medium saucepan 1/4 full with water. Set the saucepan over medium heat, and bring water to a simmer.
Combine egg whites, sugar, and cream of tartar in the heatproof bowl of electric mixer; place over saucepan. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes. Test by rubbing between your fingers; egg whites should feel smooth.
Transfer bowl to an electric mixer fitted with whisk attachment, and beat, starting on low speed, gradually increasing to high until stiff, glossy peaks form, 5 to 7 minutes. Add vanilla, and mix until combined.
In the same bowl, add black food coloring, one drop at a time. Beat with the whisk attachment until color is incorporated. Continue adding drops and beating until meringue reaches deep black color. Be sure to use meringue immediately.