Just a quick note to clarify a few points of confusion and address several comments and questions:
1) I blend the broccoli to a pulp before adding it to the brownie mix. I use it instead of butter.
2) Litlove, I doubt it would fool a 14-year-old. But try it, you never know. Spinach also works - in fact, I kind of like spinach brownies. And you can tell him it's a pot brownie.
3) Maven, good point - plain old broccoli would be far less disgusting but there you have it. There's no accounting for taste.
4) And a final note about K - she liked fruit and tolerated veggies until she turned 3 and then overnight refused to eat them any more. Juice and yogurt, too. Her explanation (now that she can talk) is that she doesn't like wet food, although she likes ice cream just fine. Go figure. I can usually compel her to eat a small portion of carrots or cauliflower if she wants dessert but she won't snack on apples, grapes and bananas like other kids. But she's healthy so I don't worry, I just content myself with creating really bad memories of her mom's baking skills so she can, I dunno, roll her eyes at me in high school or something.
And if you want more sneaky recipes, check out Jessica Seinfeld's cookbook Deceptively Delicious.