The cake is spoken of frequently throughout Portal. It is often mentioned through dialogue as well as Rattmann's wall art in his dens. Although the dens frequently claim that the cake is in fact a lie, the ending of Portal shows the actual cake. This implies that GLaDOS really hid the cake very well for "Cake and Refreshments". The cake is also mentioned on the monitors in the escape sequence (in the midst of gibberish, you will find some cake recipes mixed).
Recipe In addition to the simple mention of the Cake, an actual recipe is given. The recipe can be found on several computer screens after Chell's escape from death. Additionally, during the battle with GLaDOS, the Intelligence Core will recite the recipe for the Cake. The recipe often contains unusual (e.g. "candy coated peanut butter pieces, shaped like fish") or potentially lethal (e.g. "cranial caps") ingredients.