The Black Forest cake, alternatively referred to as the "Black Forest gateau” or simply "cake", typically consists of layers of chocolate, with whipped cream between layers, cherries on top, and sprinkled with icing. However, some Black Forest cakes alternate with a covering of chocolate shavings, such as the type produced by Aperture Science.
For a time in the Aperture Science computer-aided Enrichment Center, Black Forest cake used as a motivational object for Test Subjects utilizing the Aperture Science Handheld Portal Device. After GLaDOS's takeover in May 200- however, she incinerates Test Subjects after the completion of the test, negating the need for the cake as a reward, but still uses it to motivate Test Subjects.
Upon Chell's awakening over twenty years after the facility was taken over, GLaDOS makes her navigate Test Chambers utilizing the Portal Gun, with the promise of cake and grief counseling at the conclusion of the test. Upon arrival at Test Chamber 16 however, Chell discovers desperate messages scribbled on the wall by former Aperture employee Doug Rattmann, who went mad during his captivity, and was convinced that the "cake is a lie". Upon
the completion of the test, she is greeted with an incinerator, apparently confirming that the cake is indeed “a lie”. While making her way through the abandoned facility, GLaDOS tries to convince her to turn back by telling her "somebody cut the cake" without waiting for Chell. However, GLaDOS later states that she is going to kill Chell and that "all the cake is gone". Nevertheless, after Chell partially destroys GLaDOS and taken by the Party Escort Bot, GLaDOS activates a series of Personality Cores, while at the same time extinguishing the candle on the cake, which was not a lie after all. Rattmann is not entirely wrong: while the cake itself exists, the lie resides in receiving the cake after the tests.
GLaDOS has a strong affinity for cake, often unnecessarily mentioning it, and even having her Intelligence
Core and the computer screens in the labs reciting her personal cake recipe. That recipe has both safe, non-toxic ingredients and strange, dangerous additives. The player can find it written on many of the screens around the office areas, but it is interspersed with random numbers and letters, making it difficult to read.