Preheat the oven to 225 degrees F. Line a large baking sheet with parchment paper or non-stick mat, such as a silpat.
In a very clean large bowl, using very clean beaters, beat the egg whites, cream of tartar and salt until soft peaks form. Beating, gradually add the sugar and vanilla until stiff peaks form and the mixture becomes glossy. Fold in the cocoa powder and transfer to a pastry bag with a plain tip. Pipe the meringue into large circles onto the prepared baking sheet, making the sides higher than the center to make a bowl-shape for holding the mousse and leaving 1-inch space between each. (Alternatively, drop the meringue with a spoon onto the baking sheet and form large circles, pressing with the back of the spoon to make indentations in the centers.) Bake on the middle rack of the oven for 1 hour. Turn off the heat and leave in the oven until the meringues are dry and the tops have slightly cracked, about 1 hour.
Cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely. Serve at room temperature. (The meringues can be made in advance and kept in an airtight container at room temperature for up to 2 days.)
Yield: 6 servings