A biscuit was a baked flour-based food often served with tea. (TV: The Invasion, The Last Precinct)
Types of biscuits
The Second Doctor once asked for a patty cake biscuit with his tea. (TV: The Invasion)
Kirsty McLaren offered a wheat biscuit to Polly Wright, who mistook it for a dog biscuit. (TV: The Highlanders)
Sailors often ate hardtack, which they did not like. (TV: Enlightenment)
The Best company sold biscuits. (TV: Liberation)
The Seventh Doctor's favourite biscuits were chocolate HobNobs. (PROSE: Cold Fusion) He had a biscuit recipe that had been given to him by Pliny the Elder. (PROSE: The Hunting of the Slook)
The Tenth Doctor claimed that custard cream biscuits and Garibaldi biscuits were "the best". (PROSE: Code of the Krillitanes)
The Eleventh Doctor hated ginger biscuits (PROSE: Dead of Winter) and Bourbon biscuits (PROSE: Chasing the Dawn) but was very fond of Jammie Dodger biscuits. (TV: Victory of the Daleks, The Bells of Saint John)
The Thirteenth Doctor's TARDIS control room contained a feature which dispensed custard cream biscuits. The Doctor discovered it when investigating her new TARDIS interior, and it was operated by a foot pedal. (TV: The Ghost Monument) However, like her eleventh incarnation, she disliked Bourbon biscuits. (PROSE: Chasing the Dawn)
Other references
Kady Williams liked store-bought biscuits. She said that this was because she was still "an Earthling" at heart, even after her years of travel as host of the Gestalt. (PROSE: Ring Theory)