Make cake: Line a 9-inch loaf pan with plastic wrap; tuck it into corners and leave a 1-inch overhang. Mix 1/2 cup cream, marshmallows and espresso powder in a saucepan. Cook over medium-low heat, stirring, until marshmallows melt and espresso dissolves. Remove from heat; whisk in chocolate until melted. Scrape into a bowl; let cool to room temperature, whisking occasionally, 20 to 25 minutes.
Pour remaining 1 1/2 cups cream into a bowl; whip with an electric mixer until soft peaks form. Fold a cup of whipped cream into chocolate mixture. Gently fold chocolate mixture into remaining whipped cream, taking care not to deflate cream. Arrange 3 cookies in a single layer on bottom of pan. Scrape 1 cup of mousse over cookies and smooth with an offset spatula. Stand one cookie up on each short end of pan.
Arrange 3 cookies on top of mousse. Spread quarter of remaining mousse evenly over cookies. Repeat with remaining ingredients, forming 5 layers of mousse and ending with cookies. Cover pan with plastic; freeze for at least 6 hours.
Make sauce: Melt chocolate and 8 Tbsp. water in a bowl set over pan of simmering water. Stir in liqueur, if desired. Serve cake with sauce.