If you like your brownies like I do--dense and fudgey, as opposed to cakey and light--the following recipe is for you.
Interestingly enough, I came across said recipe when I bought a 9-inch-by-9-inch baking pan at Williams-Sonoma some years ago.
I've mixed in all sorts of ingredients--chocolate chips, M&Ms, Reese's Peanut Butter Cups, Thin Mints--since then, but the mix-in mentioned below--Reese's Pieces--may be my favorite.
Reese's Pieces Brownies
16 tablespoons (2 sticks) butter
6 ounces unsweetened chocolate, finely chopped
2 cups sugar
Pinch of salt
4 eggs, room temperature
2 teaspoons vanilla extract
1 1/2 cups all-purpose flour
1 1/2 cups Reese's Pieces (or the "mix-in" of your choice--chocolate chips, M&Ms, Thin Mints, etc.)
Before you do anything else, preheat your oven to 350 degrees. After that, line a 9-inch square baking pan with lightly greased parchment paper.
Once that's done, toss the butter and the chopped chocolate into a medium saucepan set over low heat. Stir the concoction until completely melted. Take the pan off of the heat and stir in the sugar and the salt, then add the eggs and vanilla and stir until well blended. Finally, add the flour and stir just until it has been incorporated. Stir in the Reese's Pieces.
Pour the batter into the prepared pan and smooth the top. Bake until a toothpick inserted into the center comes out almost completely clean, 40-45 minutes. Don't overbake or the brownies will be crumbly and dry. Transfer pan to a wire rack and cool completely before cutting into squares.