You can use the adjective half-baked to describe your sadly underdone cupcakes, or in a figurative way to criticize your brother's crazy business idea.
When something's half-baked, it's just never going to work. If your plan for moving to Iceland is half-baked, it means you haven't thought the whole thing through. This metaphorical meaning came from the original definition of half-baked, literally "baked halfway" or "underdone." If something's half-baked, nobody wants to eat it — it's useless. An idea or plan, likewise, is half-baked if isn't worth wasting time on.