Try 1 1/2 cup of the cream soda to sub for all the liquid - eggs and milk. Baking powder reduce to 1 1/2 tsp. From my quick calculations I'd cut butter/margarine down to just under 1/4 cup, but your recipe might just be richer than the one I'm looking at.
The yields don't quite work out for me though. I'm downsizing from a bulk recipe. I won't guarantee success (baking is really temperamental when it comes to quantity), but those are the changes I'd try.
EDIT: 1 1/2 cup = 12 oz bottle (easier than measuring it!)
Subbing cream soda for the root beer and vanilla for the root beer extract. Topping wise maybe a caramel buttercream or even just a nice drizzle? I never frost cupcake or cakes unless I'm giving them to someone else.
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