A Few Quick Notes Before You Start
Tossing in a cup of whatever rogue ingredient strikes your fancy is an easy and fun way to experiment in the kitchen (and is a lot less stressful than, say, experimenting with soufflés). Whichever goody you're baking, fold the mix-ins into the batter last, just before spreading into the pan (brownies) or dropping by spoonfuls onto sheets (cookies). Also, it's easier to cut brownies that have chunky additions (like nuts or candies) into squares if you line the pan with foil before pouring in the batter; once they're baked, chill the entire pan before you lift out the brownies and slice them. As for cookies, if you can stand the wait, chill the dough for 24 hours before baking. The cookies will spread evenly when they bake, won't get thin around the edges and will be thicker and chewier all over.
The Nutty, Salty Crunch
Chopped nuts are perennial favorites in both brownies and chocolate chip cookies. While walnuts are traditional, hazelnuts (which, combined with chocolate, impart a Nutellaesque flavor) and almonds (think rocky road ice cream) deserve to be in the rotation too. Honey-roasted nuts and salted corn nuts are excellent in cookies (though they don't work quite as well in brownies). Rice Krispies add crunch to either dessert, and mini pretzels, potato chips and even Cool Ranch Doritos do that plus deliver a salty kick. Other ideas: wasabi peas, cheese puffs or Fritos. The basic rule of thumb with this type of mix-in is to incorporate 1 to 1 1/2 cups per batch.
The Fruity, Sweet Hit
Small candies such as Reese's Pieces or toffee bits are always winners, but if you like a more subtle sweetness, consider chunks of dried fruit, like apricot or cranberry, or candied versions, such as ginger or orange. Flakes of sweetened coconut are fantastic too. You can even put fresh fruit in brownies: Pour half the batter in the pan, stud it with raspberries or pitted cherries, and cover with the remaining batter. Again, 1 cup is a good ballpark amount.
The Stealth Flavor Makers
Powders and zests won't change the texture of the finished brownie or cookie much, but they'll radically transform the taste, even when you use them in small amounts. Stir in 1 to 3 teaspoons of cinnamon, chili powder (ancho is particularly nice; it has a mild heat and fruitlike sweetness), ground or freshly grated ginger, or instant espresso powder. Lemon or orange zest also works well—especially in cookies that contain dark chocolate instead of semisweet.
The Finishing Touches
You can press any of the mix-ins we've mentioned—from Fritos to bits of candied orange—on top of the brownies or cookies before they go into the oven for a "ta-da!" presentation. This is also a good place to put more fragile additions—such as popcorn—that would probably disintegrate if you stirred them into the batter. Another easy topping trick is to pull brownies out of the oven a minute or two before you think they're done cooking and dot the surface with mini marshmallows; then return the pan to the oven. It won't take more than two minutes for them to melt enough that you can spread them with a spatula. Let cool before serving or storing.
Next: 7 of the best brownie recipes ever