Some variations! Throw in a two-inch sprig of rosemary with the butter to make a rosemary butterscotch (and just remove the rosemary from the sauce after it simmers). Add in ½ teaspoon ground ginger, ½ teaspoon cinnamon, ¼ teaspoon ground cloves, ¼ teaspoon all spice, and a pinch black pepper for a gingerbread twist.
I find that this butterscotch version is best enjoyed the day that it’s baked, but other versions with red bean, black sesame, peanut butter, and chocolate will keep for at least two days in the Pyrex dish at room temperature, and longer in the fridge (though you’ll want to heat it briefly before serving).
Find it online: https://tworedbowls.com/2018/12/14/butterscotch-mochi/