Preheat oven to 160°C. Heat kettle of water to boiling; remove from heat.
Place chocolate in a small heatproof bowl. In a large bowl, whisk egg yolks, eggs, brown sugar and salt until blended. In a 2,8-litre saucepan, heat milk and ½ cup cream on medium until bubbling. Pour enough over chocolate to cover; let stand. Slowly whisk remaining hot milk into egg mixture. Gently whisk chocolate mixture until smooth; pour into milk-egg mixture, while whisking, until smooth. Whisk in espresso or coffee and 1 teaspoon vanilla. Skim off and discard any bubbles or foam on surface of mixture.
Divide equally among 8 ramekins or ovenproof cups; place in roasting pan and put in oven.
Pour enough water from kettle into roasting pan to come halfway up sides of ramekins. Cover pan loosely with foil. Bake custards for 25 to 30 minutes or until knife inserted halfway between edge and centre of custard comes out clean.
Remove foil. Cool ramekins in pan on wire rack until warm; transfer to wire rack to cool. Cover with cling wrap; refrigerate for 3 hours or up to 3 days.
When ready to serve, in a small bowl, stir instant coffee powder into unused vanilla until dissolved. In a large bowl, beat the remaining ½ cup cream until thickened. Add white granulated sugar and beat until soft peaks form. Fold in vanilla mixture. Dollop on top of custards and serve.