Steps

Step 1: Make the crust: Line a 9-inch-by-13-inch baking pan with parchment paper, leaving a 2-inch overhang on 2 sides. In a food processor, or in a resealable plastic bag, crush the graham crackers until you have fine crumbs (but stop before you have dust). You should have about 2 1/4 cups. Transfer the crumbs to a medium bowl. Add the butter, sugar and salt and stir until evenly moistened. Tip the crumbs into the prepared pan and press them down into an even layer on the bottom. Transfer to the freezer while you prepare the filling.

Step 2: Make the filling: Set the Duke’s Chocolate Bar in a medium bowl. In a small saucepan, heat 1 cup cream, espresso powder and salt until hot but not boiling. Pour the hot cream mixture over the chocolate and let it stand for 2 minutes. Add the vanilla and whisk until smooth. Set aside to cool completely.

Step 3: In a large bowl or in the bowl of a stand mixer fitted with the whisk attachment, whip the remaining 2 cups of heavy cream until you have stiff peaks. Add the chocolate mixture and gently fold to combine. Pour the mixture over the prepared crust, and spread it out into an even layer. Cover with plastic wrap and chill until firm, at least 2 hours. 

Step 4: To serve, cut the two edges without parchment free with a sharp knife then use the parchment overhang to transfer the bar to a cutting board. Cut into squares and serve with a dollop of whipped cream, if desired.