I want to substitute coconut oil for the butter in a brownie mix but I don't know how much to use. The recipe calls for 3/4 stick of melted butter. How much coconut oil should I use?
Butter is about 80% milkfat, and 20% water, more or less.
3/4 stick is 6 tablespoons.
You should be able to substitute six tablespoons of coconut oil one to one. It should be well within the tolerance of the recipe.
If you really want to account for the water, you would do about 5 tbl of coconut oil, and 1 tbl of liquid.
Butter is 83% fat, 17% water (we can ignore the milk solids in this case). Instead of 90 g butter, you can use 75 g coconut oil and 15 g more liquid (e.g. milk, or whatever the recipe specifies). There will be a slight difference in texture in the finished product, but nothing to worry about.