claisen ester condensation

when ethyl acetate is treated with sodium ethoxide or any base it forms sodium salt of ethyl acetoacetate which after work up in HCL give aceto acetic acid and beta keto ester.This reaction is known as claisen ester condensation.

Step 1: 

NaOEt is a strong base which captured one hydrogen from methyl group of ethyl acetate as a result we get CH2-COOEt .

Step 2:

Further CH2-COOEt act as a nucleophile and attack 2nd mole of ethyl acetate and we get ethyl aceto acetate.