In a small saucepan over a medium heat, combine 110 g light soft brown sugar and 30 g unsalted butter, stirring constantly until the sugar has dissolved and the butter has melted, about 2–3 minutes.
Remove from the heat, then stir in 60 ml double cream, ½ tsp vanilla extract and a pinch of salt. Whisk until smooth and leave to cool slightly.
In a medium bowl, combine 120 ml cold double cream, 30 g icing sugar and ¼ tsp vanilla extract. Whip on medium-high speed until soft peaks form.
Place four tall glasses on your work surface. Divide the butterscotch sauce between them, using about 1–2 tbsp (15–30 ml) per glass, to taste.
Slowly pour one 330 ml bottle of chilled cream soda into each glass, leaving room at the top for the cream. Gently stir if needed.
Top each drink with a generous dollop of whipped cream.
Serve immediately and enjoy your frothy, sweet Butterbeer!