Preheat the oven to 210F (100C) with the ventilation on. Draw one large heart about 7 inches (18cm) in diameter on a sheet of parchment paper. Mind that it will spread as it bakes. Flip it over so the pencil is on the bottom and place it on a baking sheet.
In a stand mixer, beat the egg whites with a pinch of salt until soft peaks form, which should take a few minutes.
On medium speed, add in the powdered sugar, a tablespoon at a time, until all is incorporated. Beat the meringue at high speed until the sugar is dissolved.
Add the cocoa powder and mix until incorporated.
At the end add in the vinegar and mix at high speed until very stiff peaks form.
Dollop the meringue gently onto the prepared parchment, and using a spatula, fill in the shape of the heart.
Put it in the oven for about 1 1/2 - 2 hours, it depends on the oven. When it's done it should lift off easily from the parchment paper. If it's sticky let it cook a bit longer. At the end of the baking time, turn off the oven and crack the oven door open a bit, and allow the meringue to sit inside the warm oven to slowly cool.