The brownies are probably coming out fluffy because of how you're mixing or what's going into the batter. Fat content matters way more than people think - if your recipe doesn't have enough butter or chocolate relative to flour, you'll get cake every time. But there's something else: are you beating air into the batter? When you cream butter and sugar or whisk eggs too much, you're incorporating air that makes them rise. Fudgy brownies need minimal mixing. Just stir everything together until combined, don't go crazy with a mixer.
I learned this the hard way making brownies at home. I switched from creaming my butter and sugar to just melting them together and stirring gently, and the texture got way denser and chewier. The other thing is your baking time - 25 minutes at 350 might actually be fine depending on your pan size, but pull them out when a toothpick inserted in the center still has some moist crumbs clinging to it. Not wet batter, but definitely not clean. They keep cooking after you take them out, so pulling them slightly early is better than waiting for that perfect done look.