Add egg whites, sugar and vinegar to a heat proof mixing bowl (I used the stainless steel bowl that comes with my stand mixer). Place this bowl over a pot of simmering water (not boiling, so that you don’t cook your egg) and whisk the egg until sugar dissolves and you can’t feel the sugar granules with your fingers. The mixture should only be slightly warm.
Remove from heat, transfer the bowl to your stand mixer and beat with the whisk attachment on medium low until foamy.
Add salt and vanilla, continue whisking on medium until stiff peak. The meringue should now be a shiny white and the tip should stand up when you flip the whisk over.
Sift cocoa powder over the meringue and gently fold it in with a spatula.
Line a large baking sheet with parchment paper, dot the bottom corners of the parchment with meringue so it doesn’t lift up.
Transfer meringue to a pastry bag and pipe your desire shape or simple spoon the meringue onto the parchment paper.
I used an open star tip and got about 3 dozen meringue cookies on the baking sheet, each cookie was about 1” – 1.5” wide and 1” – 1.5” high.
Bake in the oven for 4 - 5 hours. In the last hour, prop the oven door open with a cork trivet or wooden spoon to allow moisture to escape and to make sure the cookies can be dry completely.
Check to make sure the cookies are dry to the touch and not squishy. Turn off oven and leave them in the oven to dry throughout the day or overnight.