How to make brownies more chewy?

i am using a boxed brownie mix from betty crocker. it calls for 3 tbsp water, 1/2 cup veg. oil, and 2 eggs. im also using an 8x8 pan. what should i do to make them more chewier? please tell me how long to cook them, the degrees, and how much of each ingredient i should use to make them chewy. thanks!!!