Heat the butter in a small saucepan over medium heat just until melted. Add the brown sugar and salt and mix to combine. When the mixture starts to liquify and the mixture no longer resembles wet sand, add the cream. Mix to combine. Bring the mixture to a boil, stirring once in a while to prevent the bottom from burning. Let it boil for 10 minutes or until it thickens and coats a spoon. Let it cool and stir in the vanilla extract.

Preheat the oven to 300F. Place 5-6 (depending on the size) pudding bowls or ramekins in a deep roasting pan and set aside.

In a saucepan, combine the cream, milk, soda, and butterscotch over medium heat. Meanwhile, stir the yolks, salt, and vanilla extract together and set aside.

Bring the cream mixture to a simmer and temper the yolks by slowing whisking the hot cream into the yolks. Strain the mixture through a fine-mesh sieve. Ladle the mixture into the ramekins.

Transfer the pan to the oven and pour enough hot water to come up half way up the ramekins. Wrap the top of the pan with foil and bake for 45 minutes or until the center of each pot de creme gives a slight wiggle, not a wave. Once done, transfer the ramekins out of the water to cool completely. Refrigerate until cold. It's optional, but you can top each pot de creme with a dollop of lightly sweetened whipped cream.