This no-bake chocolate cake is dense with dried cherries, chopped walnuts, and crumbled whole-wheat biscuit cookies.
Lightly butter an 8-by-2-inch spring-form pan; set aside. In a medium bowl, combine chocolate and butter. Place bowl on top of a saucepan filled with 1 inch of simmering water. Stir mixture occasionally until melted and smooth, about 3 minutes. Remove bowl from saucepan; set aside.
In another medium bowl, whisk together egg, sugar, and water. Place over the pot of simmering water; whisk until pale, fluffy, and thick, 3 to 4 minutes. Remove bowl from heat; whisk in chocolate and rum, if using. Let cool for 5 minutes, stirring occasionally.
Fold crushed cookies, half the walnuts, and half the cherries into chocolate mixture. Pour into prepared pan; smooth top with a spatula. Sprinkle remaining nuts and cherries on top. Cover; chill until set, about 1 hour. Serve with whipped cream, if using.