If you are looking to mimic the moisture of oil while using butter, you might consider "browning" the butter. If a recipe calls for melted butter and you decide to use oil, you generally need to use slightly less oil.
Why Oil Brownies Stay Soft Days Better Than Butter Versions
A standard conversion is to use about 20% less oil than the amount of butter specified. This process removes the water content, bringing the fat ratio closer to that of oil while simultaneously adding a rich, nutty flavor that elevates the brownie significantly.
Oil: Superior moisture retention, leading to a dense, fudgy texture. To brown butter, melt it slowly in a pan until the milk solids turn golden brown and settle at the bottom.
Why Oil Brownies Stay Soft Days Better Than Butter Versions
Oil is a pure fat, meaning it is 100% lipid with no water content. When butter is melted, the water content hydrates the flour differently and creates steam during baking, which contributes to lift and a slightly cakier texture.
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