Katakana キケン (Kiken) is actually kanji 危険 (Kiken) which means 'Danger/Dangerous'. So it's not cake'n.
Why is it written in Katakana? Because it's a form that has a serious/strong meaning. So Tachibana actually really meant it that no one absolutely shouldn't touch Gou-chan's cake except himself.
http://www.mangago.zone/read-manga/kijima_kun/mf/tr_chapter-18/pg-19/
Katakana キケン (Kiken) is actually kanji 危険 (Kiken) which means 'Danger/Dangerous'. So it's not cake'n.
Why is it written in Katakana? Because it's a form that has a serious/strong meaning. So Tachibana actually really meant it that no one absolutely shouldn't touch Gou-chan's cake except himself.