I'd start with the simplest, cheapest fix, which is the spark plugs. Change the cylinder 1 plugs with fresh ones and see if the problem persists. If so, then switch the coils from cylinder 1 to 2 to see if the problem follows to the new cylinder or it stays.
If it stays the same no matter what you do, then it can be either the HPFP or the injector.
'08 AW E90 335i
PROcede V5 | BMS DCI | RR DPs | ETS FMIC | FORGE DVs | Stett CP | Vanguard | PSS10