1. Preheat oven to 375°F. Prepare, bake and cool cake following package directions.
2. Melt marshmallows and milk in heavy saucepan over low heat. Remove from heat; stir in milk chocolate chips until melted. Cool to room temperature. Beat whipping cream in large bowl until stiff peaks form. Fold cooled chocolate mixture into whipped cream. Refrigerate until of spreading consistency.
3. To assemble, split cake horizontally into 3 even layers. Place 1 cake layer on serving plate. Spread with one-fourth of frosting. Repeat with second cake layer and one-fourth of frosting. Top with third cake layer. Frost side and top with remaining frosting. Refrigerate.
4. For drizzle, place semisweet chocolate chips and shortening in 1-cup glass measuring cup. Microwave at MEDIUM (50% power) for 1 minute. Stir until smooth. Drizzle melted chocolate around outer top edge of cake, allowing mixture to run down side unevenly. Refrigerate until ready to serve.
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