Recursive running totals

List.Generate is the best way to use for running total and you use it in the best way but do not use it as a custom function and use it directly as a part of your code, additionally instead of using List.Sum like List.Sum({[ RunningTotal ], AmountBuffer{[Counter]+1}}), use simple sum operation as 

[ RunningTotal ]+ AmountBuffer{[Counter]+1}

to make it more efficient and if you whant to apply it over several columns do it at once by adding field value over List.Generate