Preheat the oven to 225°F (107ºC). In a medium mixing bowl of a stand mixer fitted with the whisk attachment (or using a handheld mixer), beat the egg whites with cream of tartar on high speed until foamy.
Gradually add the sugar, 2 tablespoons (25g) at a time, beating consistently until the sugar is completely dissolved and the egg whites are glossy and stand in stiff peaks. If desired, gently fold in chocolate chips.
Drop round spoonfuls of meringue batter (if added chocolate chips) or pipe through a pastry bag, 1-inch (2.5cm) apart on waxed paper on a cookie sheet.
Bake until the meringues are firm and they easily lift off of the waxed paper, about 1 hour.
Turn off the oven and let the cookies stand in oven with door closed until cool, dry and crisp, for at least 30 minutes to an hour.