Impossible soufflé grills were a product sold at the Übermarket. At the time the Twelfth Doctor, Bill Potts, and Nardole visited the store, an offer was taking place: buy one packet and get another two free. (COMIC: The Great Shopping Bill)
Behind the scenes[]
The name appears to be a pun on both "impossible girl" and "soufflé girl", two nicknames coined by the Doctor for Clara Oswald.