Mocha Icebox Cake

It's hard to believe that you only need 6 ingredients to make this delicious mocha iced cake. If desired, you can fill the chocolate sauce with coffee liqueur!
Prepare the cake: Cover a 9-inch baking dish with plastic wrap; roll it up at the corners and leave a 1-inch protrusion. Combine 1/2 cup cream, marshmallows, and espresso powder in a saucepan. Cook over medium-low heat, stirring occasionally, until the marshmallows are melted and the espresso is dissolved. Remove from the heat; beat the chocolate until it melts. Transfer to a bowl; let cool to room temperature, whisking occasionally, 20-25 minutes.Pour the remaining 1 1/2 cups cream into a bowl; beat with an electric mixer until soft peaks form. Add a cup of whipped cream to the chocolate mixture. Gently mix the chocolate mixture with the remaining whipped cream, being careful not to blow the cream away. Place 3 cookies in a single layer on the bottom of the pan. Apply 1 cup of mousse to the cookies and smooth them out with a spatula. Place one cookie on each short edge of the pan.Place 3 cookies on top of the mousse. Spread a quarter of the remaining mousse evenly over the cookies. Repeat with the rest of the ingredients, forming 5 layers of mousse and finishing with the cookies. Cover the pan with plastic wrap; freeze for at least 6 hours.Make the sauce: Melt the chocolate and 8 tablespoons of water in a bowl set over a pot of boiling water. Add liqueur if desired. Serve the pie with the sauce.