Brownies, unfortunately, will never be considered a health food, but there are ways to minimize the amount of unhealthy trans fats in the chocolate treats. Substituting olive oil for use in brownie mixes is one way, though olive oil selection is a key in preserving the flavor of the brownies. More adventurous cooks can make olive-oil-based brownies from scratch, resulting in a savory treat free of some of the unhealthy fats found in vegetable oil based margarine.
If baking brownies from a box, choose a light or extra light olive oil. These olive oils are heavily refined and do not have as strong a flavor as other olive oils, so they will not clash with the taste of the brownies.
Use the same amount of olive oil as the amount of vegetable oil called for in the package instructions. Follow all package instructions as written.
Experiment with a test batch if you're not sure of the flavor strength of your olive oil. Divide the specified amount of vegetable oil in half, and use a 50-50 mixture of the olive oil and a milder oil. Canola oil is a good choice if you're trying to keep trans fats as low as possible.
Consider throwing out the box and making a batch of brownies from scratch. These recipes often call for butter or margarine--which is made of vegetable oil--and allow for more flavorful experimentation with olive oils.
Substitute olive oil for butter or margarine at the rate of 3/4 of the amount of butter or margarine called for in the recipe. This means 1 tsp. is equal to 3/4 tsp., 1 tbsp. is equal to 2 1/4 tsp. and 1 cup is equal to 1/4 cup.