I'm looking to replace the butter in my brownie recipe with oil. Is it a 1:1 ratio? If not what is the substitution ratio?
Replacing Butter with Oil
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It's not a simple replacement necessarily. Besides the fat, there's the flavor of butter, the sugars and so on in the butter.
Quantity wise, butter is 15-20% water so you might want to reduce the oil and add water. But also it depends on how butter is used in the recipe. If you're melting the butter and waiting until the foaming stops, that means you've evaporated most of the water. If you're creaming the butter first with the sugar, that's a step that affects leavening, texture and such in the final baked good. You can't replicate that with a simple oil-butter swap.
You'll need to post your recipe and technique for us to give a better answer.