Melt brown sugar, butter, chocolate, and egg in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching.
Stir graham cracker crumbs and walnuts into chocolate mixture; press evenly into the bottom of the prepared pan.
Stir together shredded coconut and condensed milk in a bowl; spread evenly over chocolate layer. Spread frosting over coconut mixture. Refrigerate until ready to serve.