1.Preheat oven to 120°C. Line three large oven trays with baking paper.
2.Stir coffee and the water in a small bowl until dissolved. Beat egg whites and cream of tartar in a large bowl with an electric mixer until soft peaks form. Gradually add caster sugar, beating until dissolved between additions. Beat in cornflour, vinegar and coffee mixture on low speed until just combined (overbeating at this stage will cause the meringue to deflate).
3.Drop ¼-cups of meringue mixture onto trays 5cm apart; you should get about 24 meringues. Bake for 1 hour or until dry to touch. Turn oven off, leave meringues to cool in oven with door ajar.
4.Make chocolate sauce. Stir chocolate, butter and cream in a medium saucepan over low heat until just melted. Remove from heat; gradually whisk in sifted icing sugar. Cool to room temperature (about 20 minutes) until thickened.
5.Make coffee cream. Stir coffee and the water in a small bowl until dissolved; refrigerate 5 minutes. Beat thickened cream, sugar, liqueur and coffee mixture in a small bowl with an electric mixer until soft peaks form. Add thick cream; beat until soft peaks form.
6.Spoon a little coffee cream onto a platter, arrange nine of the meringues on top. Spread the base of each remaining meringue with 2 tablespoons of coffee cream, stacking them into a pyramid shape. Drizzle stack with half the chocolate sauce, then dust with icing sugar. Serve stack immediately with remaining coffee cream and sauce.
Meringues can be made 2 days ahead; store in an airtight container at room temperature. Chocolate sauce can be made several hours ahead; reheat until just warm before serving. Assemble meringue stack close to serving.Note