Melt the butter in a medium saucepan over medium heat. Stir in the brown sugar, salt, and cream of tartar. Cook for 1 to 2 minutes, until the sugar melts and the mixture comes to a boil.

Carefully pour in the heavy cream, stir well, and boil for 3 to 5 minutes. The sauce should thicken and coat the back of a wooden spoon.

Take the pan off heat and stir in the vanilla extract. Set aside while you prepare the drink.

At least 3 hours ahead, pour 2 cups of cream soda into ice cube trays and freeze until solid.

Place the frozen cream soda cubes, vanilla ice cream, 1 cup cream soda, and optional flavors into a blender. Blend until smooth, stopping as needed.

Divide the frozen butterbeer among 4 glasses and top each with about 1/4 cup whipped cream and 2 tablespoons of butterscotch sauce. Serve immediately.

Gently stir the 3 cups cream soda with vanilla extract and optional extracts in a large pitcher. Mix lightly to avoid losing fizz.

Pour into glasses, spoon on whipped cream, and drizzle with butterscotch sauce. Serve immediately.

For adults, you can add a shot of scotch, rum, bourbon, vodka, or schnapps to each serving.

If you have leftover butterscotch sauce, cool and store it in a lidded glass container in the refrigerator for up to 2 months.