These have been a staple of life for my family, friends, picnics, and camping trips for several years. It took a few tweaks to get the formula just right; since then I can practically make them in my sleep. And probably have.
If you want to use white chocolate chips rather than semisweet, go right ahead. A mixture of both? Well, why not. Add nuts? Sure. Especially if they're macadamias. Just be sure to toast your nuts (350 degrees until fragrant) first for that great extra layer of flavor.
Another wonderful variation is to reduce the vanilla extract to 2 teaspoons and add a teaspoon (or more - give the batter a taste) of mint extract, though I'd probably omit any nuts here. Espresso powder dissolved in your vanilla extract - heavenly. A couple of ounces of Kalhúa - oh yeah. The sky may not be the limit, but it's close. And for fall, scoop out a cup of the finished batter; mix in a half cup of canned pumpkin; scrape the un-pumpkinned batter into the prepared baking pan, then splop in the pumpkin batter; use the end of the spatula handle to marble it in. You'll probably need to extent the baking time by 5-7 minutes.
A note on baking and cutting. These beauties are dense - no leavening - so they require some patience before cutting. I find they cut best if made the day before I plan to serve them. Once they're cool, simply wrap the pan well with plastic, store at room temp, then depan and cut the next day after they've had a chance to set up.
And a note on the baking pan. If you use a smaller one, your brownies will take longer to bake by 5 or 10 minutes because they are more dense. If you use a larger one, say a 10" round cake pan as you are welcome to do, they will bake more quickly because of being thinner and less dense, say 5 to 7 minutes less. If you use a 9"x12" pan, you're on your own because they will too thin. They will bake too fast and your family/guests will wonder why they are holding a cookie rather than a brownie.
These freeze very well. During the summer especially, it's nice to have a so to speak rabbit to pull out of a hat without having to scramble for ingredients or turn on a hot oven.
Finally, a note on the healthy option. Cut smaller portions. Go for a hike.
No rabbits were harmed in the making of these brownies.
Preheat oven to 325 degrees. Line a 9" x 9" baking pan with parchment and grease or spray the sides.
Measure chopped semi-sweet chocolate, unsweetened chocolate, cocoa powder, and butter into a large heatproof bowl and set over simmering water. By melting the cocoa powder into the other chocolates and butter, you get a much better mix with it, and its flavor is deeper. Stir occasionally until all ingredients have melted. Remove from heat and stir to thoroughly blend.
While chocolates are melting, whisk together the eggs, sugar, and vanilla extract. When chocolate mixture is ready, whisk in the egg mixture. Set or hold a sieve over the bowl and measure the flour and salt right into it. Sift into the bowl, pressing any lumps through with your fingertips. Use a good stiff spatula to stir everything together just until blended.
Now gild the lily. Add the chocolate chips of your choice and your nuts if using them. Once everything is nicely blended, scrape the batter into your pan. Spoon in peanut butter if using, and use the tip of the spoon to swirl it in. Bake for 30-35 minutes. When done, the surface should be shiny and the brownies still tender to the touch. A toothpick test is useless; if you bake them until one comes out clean, they'll be dreadfully overbaked, and that would be so sad.
When done, cool on a rack until completely cool. Wrap securely with plastic and leave at room temperature overnight. To depan, run a small spatula around the edge and invert the pan onto a sheet of parchment. Place a cutting board larger than the brownies on top of the bottom. Carefully turn right-side-up. Cut into 12 equal pieces, or more. They're fantastic served slightly warmed with your favorite ice cream.