As Cat says, it doesn't work that way. The carbs you eat are the carbs you eat. And, while fiber will, in most cases, modify the absorbable fiber in food it's part of, it does hunt down the carbs from other foods, as it were.

If you really need brownies, find a good low carb recipe for them. But if you find yourself craving sweets, to me, the worst thing to do is to eat sweets. Eating anything else that IS on plan, is better because it doesn't reinforce that addictive area of the brain.