Vegetable oil is a blend of oil that may include canola and soybean oils and features a neutral flavor. Though pure fat, vegetable oil contains far lower levels of saturated fat than butter. Saturated fat imparts flavor, and butter delivers a richness to baked goods that its counterpart lacks. While vegetable oil contains only fat, butter includes protein and sugars as well, which creates even more depth in brownies. Because the butter consists of more than just the fat necessary for the batter, you will need to add slightly more butter to the mix than you would vegetable oil to ensure that there is the proper amount of fat.

When replacing vegetable oil with butter in a brownie mix, use unsalted butter to prevent an overload of salt. If you only have salted butter on hand, reduce the amount of salt called for in your recipe by at least half. Add the salt in small amounts until the level reaches your taste. Including the full amount of salt plus salted butter may make the brownies unpalatable.

Because substituting butter for vegetable oil requires you to add a larger amount of the ingredient to maintain fat levels, you may find that the batter is extra moist or even slightly thin. To counteract this effect, add two to five minutes of baking time to your recipe. The additional protein in the butter may heighten the Maillard reaction -- the carmelization-like effect of heat on protein -- and create a slightly crunchier crust. Watch the brownies closely in the final minutes of baking to ensure that they don't overcook.

In general, you should melt the butter that you will be adding to your batter so it mimics the structure of the vegetable oil it replaces. This helps create a similar crumb and texture to the original recipe. If you only soften the butter, you will need to cream it with the sugar to mix it properly. This will create more air pockets and result in brownies with a cake-like crumb. When you use melted butter, allow it to cool before adding it to your batter so it does not begin to cook the eggs.