Covertly it lurks in the refrigerated case, concealed among the soft drinks and ice teas. The double agent of desserts, it is rich yet somehow light and not off-puttingly eggy. A quivering homemade creation that is simultaneously rico and suave. See, the real secret at the Secret Sandwich Co. is not the sandwiches but the desserts, especially the flan. And now that we've told you, we'll have to kill you.