Pinkie wants to bake a cake from an ancient recipe book. Twilight is her co-pilot and taste tester on this flight of fancy. Everypony regrets it.
Fair warning, this is silly. I squeaked this out because I felt cheeky, so please don't make a stink about it. It's mostly hot air, anyway.
For those interested, Brassica vegetables include broccoli, cauliflower, cabbage, brussel sprouts, and collard greens. They're tasty, healthy, and (spoilers) they make you poop.