Place a small saucepan over medium heat and melt the butter. Add the brown sugar and stir to combine. Add the cream and stir until fully combined. Bring mixture to a low boil/simmer and cook for about 6-7 minutes, stirring occasionally.
Remove from heat and whisk in 3 teaspoons of miso paste and the vanilla. Taste and add the last teaspoon of miso if desired (I like mine with 4 teaspoons but it's pretty salty... which is great!)
Serve over ice cream, dip fruit in it, eat it straight, drizzle it on cake... the possibilities are endless. Store the leftovers in an airtight container in the fridge, and reheat before using.