Hi Stephanie! You're certainly welcome to experiment using our blog Adding sourdough to a recipe as a guide. The tricky part is that many brownie recipes don't call for a liquid ingredient that you would be using the starter to replace, like milk or water. So you'd either need to find a recipe that does call for some liquid, (like Cakey Brownies) or be willing to lose out on some potential fudginess/tenderness by replacing some of the oil or egg through some trial and error. 

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