In a small saucepan, heat 300 ml cream until warm (or microwave in a small jug until warmed). Remove from heat and set aside.
Combine butter and brown sugar in a medium stainless steel saucepan over low heat. Simmer 2 minutes, stirring with a spatula until bubbly. Add warmed cream and stir until smooth.
Combine cornflour and salt in a small bowl. Slowly whisk in 1/4 of the cold milk until cornflour dissolves. Add this mixture to the pan with remaining milk. Bring to a simmer over medium heat, stirring constantly to prevent burning. Stir about 2 minutes until you see steam rise from the surface and the pudding thickens. Slowly stream in egg yolks, whisking constantly.
Switch to spatula and continue to gently stir until bubbles form on top of pudding.
Remove from heat and stir in 2 1/2 tsp vanilla, rum (if using) and the vinegar. Quickly strain into a jug , to make pouring easier, and then immediately pour into 6 individual serving dishes or glasses.
Tightly cover each one with a piece of cling-film to keep a skin from forming. Chill 3 hours or until softly set. Can be made a day in advance.
To serve whip remaining cream with 1/2 tsp vanilla until soft peaks form. Place a dollop of the whipped cream on top of each pudding. Garnish with a toffee shard or serve with sliced bananas for that retro childhood pudding experience!