Preheat the oven to 225º F.

In a medium heat-proof bowl or the bowl of the standing mixer, combine egg whites, sugar, cream of tartar and salt. Set the bowl over a pot of barely simmering water and cook, whisking constantly, until mixture is warm and frothy and sugar is dissolved, about 2 minutes. Remove the bowl from the heat and using an electric mixer, beat the mixture on high until the egg whites are glossy and stiff, about 3 minutes. Beat in the vanilla. Fold in the cocoa powder and then the chopped chocolate.

Transfer the mixture to a medium size piping bag and pipe 30 kisses on a parchment-lined baking sheet. Bake until the meringues are dry to the touch but still slightly soft in the center, about 40 minutes. Let cool completely before removing from the parchment paper.