Follow the directions on the box of a premix exactly. Especially notice when the directions call for hot water to be used. When you use hot water, the brownies must go into the oven at once after mixing. When cold water is used, you do not have to bake at once. When you are mixing from scratch, follow the recipe exactly and if the recipe calls for hot water, place the brownies in the oven immediately after mixing.
Most recipes call for the baking powder to be added last.
Baking powder reacts to heat. Do not over mix brownies. Cream the sugar with the shortening, but mix everything else only enough to blend in. Sometime the difference between a chewy and a cake type brownie will only be the mixing time. The less you mix, the more soft and chewy the brownie will be.