Chocolate Icebox Cake

In a glass measuring cup or bowl, combine the espresso with 1 tablespoon sugar. Add 3/4 cup boiling water and stir to dissolve. Line the bottom of a 9- by 13-inch baking dish with the cookies. Drizzle the espresso mixture over the cookies; set aside. Place 2 tablespoons water in a small microwave-safe bowl. Sprinkle the gelatin over the top and let sit for 5 minutes. Meanwhile, microwave the chocolate chips and 1/2 cup cream in a medium microwave-safe bowl until the chocolate is nearly melted, 45 to 60 seconds. Microwave the gelatin mixture until it melts, about 15 seconds. Stir the gelatin into the chocolate mixture until melted and smooth. Let cool for 5 minutes. Using an electric mixer, beat the egg whites in a large bowl until soft peaks form. Gradually add 1/4 cup sugar, beating until stiff, glossy peaks form, about 4 minutes. Fold the chocolate mixture into the egg white mixture. In a second large bowl, using an electric mixer (with clean beaters), beat 2 1/2 cups cream until soft peaks form. Fold the whipped cream into the chocolate–egg white mixture. Spread over the cookies and chill for at least 4 hours and up to 3 days (making sure to cover it after 1 hour). Just before serving, using an electric mixer, beat the remaining cup of cream and tablespoon sugar in a large bowl until soft peaks form. Spread over the chocolate mousse. Sprinkle with shaved chocolate, if desired.