Butter or margarine is a suitable replacement for vegetable oil in a brownie mix. Melting the butter or margarine creates the same texture as vegetable oil for the recipe.
Once melted, the butter must slightly cool so that the eggs in the batter do not begin to cook.
Butter contains saturated fat which adds flavor and richness to the brownies. It is best to use unsalted butter, but if salted butter is used, reduce the amount of salt in the recipe. Using butter or margarine moistens the batter, so it is important to add two to five minutes of cooking time. Due to the protein in the butter, the brownies may caramelize more and have crunchier crusts.