therapy
Explanation
The Joke
A therapist (or friend) is giving an impassioned speech to their client about the freedoms of adulthood: "You're an adult. You can literally make a sheet cake -- a huge sheet cake with frosting and everything -- and eat only sheet cake for dinner. When it's gone, you can just make another. You can do this forever because you're an adult, and who's going to stop you?" The client, inspired, declares: "By God, you're right. I'm gonna go out and do it!"
The caption delivers the punchline: "'Cake therapy' outperforms cognitive behavioral therapy in all categories pertaining to eating a lot of cake." This is a parody of how psychological study results are reported, and the joke is that the comparison is absurdly rigged -- of course a therapy that tells you to eat cake will outperform a therapy that does not, specifically in the category of eating cake.
The Humor
The humor works on several levels. First, there is the absurdity of framing "just eat a ton of cake" as a legitimate therapeutic approach that could be compared to CBT, one of the most well-established and evidence-based forms of psychotherapy. Second, the qualifier "in all categories pertaining to eating a lot of cake" is a perfect parody of how scientific studies can produce technically true but completely misleading findings by cherry-picking their outcome measures. The comic satirizes both the self-help industry's tendency to dress up simple indulgence as profound wisdom, and the way research findings can be framed to make anything sound impressive if you choose the right metrics.