⅓ cup sugar ⅓ cup Dutch-process cocoa powder 2 tbsp. cornstarch ⅛ tsp. salt 1¾ cup whole milk ¼ cup heavy cream 4 oz. bittersweet chocolate, finely chopped 1 tsp. vanilla 1 tbsp. dark rum
Whisk sugar, cocoa, cornstarch, and salt in a 2-qt. saucepan. Slowly add milk and cream, whisking constantly; bring to a simmer over medium heat. Cook, while whisking, until thickened and beginning to bubble at edges, 12–15 minutes. Add chocolate, and stir until smooth, about 30 seconds. Remove from heat; stir in vanilla and rum. Divide between six 4-oz. ramekins, cover with plastic wrap, and chill until set, about 1½ hours.