Butterscotch-Chocolate Parfait Pudding Cups

In a saucepan, bring 2 cups of the milk just to a boil; remove from heat. In a bowl, whisk the remaining 1/4 cup milk, yolk, vanilla, brown sugar, cornstarch, cocoa, and salt until smooth; whisk into hot milk. Cook over low heat, whisking slowly, until mixture comes to a boil. Whisk in chocolate until melted; cook 2 minutes, whisking slowly, until thick. Remove from heat; whisk in butter until melted. Scrape pudding through a fine- mesh sieve into a 1-quart measure; spoon pudding into 5- or 6-ounce glass cups. Leave at room temperature. In a saucepan, bring 2 cups of the milk just to a boil; remove from heat. In a bowl, whisk the remaining 1/4 cup milk, yolks, vanilla, brown sugar, cornstarch, and salt until smooth; whisk into hot milk. Cook over low heat, whisking slowly, until mixture comes to a boil. Cook 2 minutes, whisking slowly, until thick. Remove from heat; whisk in butter until melted. Scrape pudding through a fine-mesh sieve into a 1-quart measure; spoon over chocolate layer. Chill until set.