In a small glass measuring cup, add 3-4 tbs of butterscotch or caramel, homemade, out of a squeezy bottle or just melted down caramels from Kraft. Mix in 3 ounces of spiced rum and mix until smooth.
Fill a glass mostly up with cream soda. top with caramel/rum mixture. I think this would scale; take a cup out of a rum bottle, empty out a 250 ml caramel sauce bottle, shake to stir and measure out a bit more than you would have added to your rum highball with cream soda. Buy the highest quality of cream soda as you can, it makes a huge difference. Don’t buy the stuff with the red dye in it, go clear or a neutral brown to get the real butterbeer look. You can top this with whipped cream if you like, or it would make *the best* vanilla ice cream float.