Preheat the oven to 350 degrees F. Butter an 6-cup souffle mold (7 1/2 inches across by 3 1/4 inches high), then sprinkle the sides and bottom with 1 1/2 tablespoons of the sugar.
Melt both chocolates together over a hot water bath; cool. Beat the egg yolks, remaining 1/2 cup sugar and salt until thick and light yellow in color. Mix the cooled chocolate, heavy cream, instant espresso and liqueur into the yolk mixture until well blended and smooth.
Beat the egg whites to a stiff peak, careful not to overbeat. Mix 1/4 of the egg whites into the egg yolk mixture. Gently fold in the remaining egg whites until well blended. Pour mixture into the prepared souffle mold.
Bake on the lowest shelf of the oven until set within a generous inch of the center, 14 to 16 minutes. Serve immediately.
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