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Whisk together the egg yolks, sugar and cornstarch in a large heatproof bowl. Slowly whisk the cream mixture into the egg mixture. Place the bowl over a pot of simmering water and stir until the custard is very thick and reaches 170 degrees F on a digital thermometer. Remove the custard from the heat, stir in the instant coffee, place the bowl in an ice bath and stir until it reaches room temperature.
Cover with plastic wrap and chill in the refrigerator for 4 to 6 hours before using.