Step 1

In a large heatproof bowl, whisk together sugar, salt and egg yolks until smooth. Place over a pot of simmering water (make sure the bottom of the bowl isn’t directly touching the water or it could scorch). Cook over medium heat, whisking until sugar dissolves and mixture is warm to the touch, about 3 minutes. Remove from heat and add chopped chocolate, butter, espresso powder and vanilla and whisk until smooth and melted. Let cool to room temperature, about 30 minutes.

Step 2

Beat heavy cream in a large bowl until stiff peaks form. Whisk ⅓ of the whipped cream into the chocolate mixture until fully incorporated. Gently fold in remaining cream, just until no white streaks remain. Spoon mixture into 6 serving bowls or ramekins. Refrigerate until set, about 2 hours. Dollop with more whipped cream just before serving.