2) In a stand mixer fitted with the whisk attachment, beat the egg whites and salt until frothy, about 1 minute. With the machine on medium-high speed, gradually add the caster sugar, about one tablespoon at a time. Add the cream of tartar, vanilla essence , and espresso powder. Increase the speed to high and beat until the mixture is thick and holds stiff peaks, about 3 to 5 minutes. Using a spatula, fold in the chocolate chips. Drop 4-8 tbsp of the mixture onto the prepared baking sheet about 5cm apart. Bake for 30 minutes. Rotate the pan and bake for another 30 minutes. Turn off the oven and allow the meringues to cool completely while still in the oven, about 2 hours.
3) Store airtight in a plastic container for up to 4 days.
Cook's Note: The meringue mixture can also be placed in a pasty bag fitted with a large, plain tip and piped onto the baking sheet.