In a large bowl, whisk 2 cups milk and pudding mix for 2 minutes. Let stand for
2 minutes or until soft-set. Split cake into three horizontal layers; place bottom layer on a serving plate. Spread with half of the pudding. Repeat layers. Replace cake top. Cover and refrigerate until serving. In a small microwave-safe bowl, heat hot fudge topping; stir in remaining milk. Drizzle over cake, allowing it to drip down the sides. Refrigerate leftovers.