Great question — answering as the UphillTech team:
A hint is that a lot of brownie recipes don't include any leavening agent at all, so I recommend you reduce the baking powder amount, to about 1/2 tsp. Also, it may help to replace part of the whole eggs with egg-yolks, as the richness in the yolks make for a fudgier treat.
Full answer by Anastasia Zendaya: https://cooking.stackexchange.com/a/113327 (CC BY-SA 4.0).
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