As a food scientist with extensive knowledge in food biology and chemistry, as well as experience in analyzing recipes and food storage practices, I am confident that I can help you solve the issue with your brownies.
Based on the information provided, it's likely that the mold growth is due to improper storage conditions or ingredients. I would start by analyzing your recipe and baking process to identify any potential factors contributing to mold growth. This could include assessing the moisture content of the ingredients, the baking temperature and time, and the sealing of the container used for storage.
Once I have a clear understanding of your baking process, I will propose changes, if necessary, to prevent mold growth. This might involve adjusting the baking temperature or time, using different ingredients, or improving food storage techniques.
My goal is to provide you with simple, non-technical recommendations that will help you enjoy your homemade brownies without worrying about mold. I look forward to working with you to solve this issue and ensure your brownies stay fresh and delicious.