The classical acetoacetatic ester synthesis utilizes the 1:1 conjugate base.  Ethyl acetoacetate is however diprotic:
:CHC(O)CHCOEt  +  NaH  →  CHC(O)CH(Na)COEt  +  H
:CHC(O)CH(Na)COEt  +  BuLi  →  LiCHC(O)CH(Na)COEt  +  BuH
The dianion (i.e., LiCHC(O)CH(Na)COEt) adds electrophile  to the terminal carbon as depicted in the following simplified form:
:LiCHC(O)CH(Na)COEt  +  RX  →  RCHC(O)CH(Na)COEt  +  LiX