5 tablespoons/2 1/2 ounces unsalted butter, cut into 5 pieces
4 ounces bittersweet chocolate, coarsely chopped
2 ounces unsweetened chocolate, coarsely chopped
3/4 cup sugar
2 large eggs
1 teaspoon pure vanilla extract
1/2 teaspoon instant espresso powder (optional, but really good)
1/4-1/2 teaspoon salt (according to taste - I used 1/2 teaspoon)
1/3 cup all-purpose flour
1 cup chopped walnuts
Center a rack in the oven and preheat the oven to 325 degrees F. Line an 8-inch square pan with parchment paper, or spray with baking spray.
Set a heatproof bowl over a saucepan of simmering water. Add the butter and chocolates to the bowl and stir occasionally until the ingredients are just melted - you don't want them to get so hot that the butter separates. Remove the bowl from the pan of water.
With a whisk, stir in the sugar. Don't be concerned when your smooth mixture turns grainy. One by one, whisk in the eggs. Add the vanilla and give the ingredients a vigorous whisking (this really changes the texture a lot) before gently stirring the espresso, if you're using it. Switch to a rubber spatula and fold in the salt and flour; stirring only until incorporated, then fold in the chopped walnuts.
Scrape the batter into the pan and smooth the top with the spatula. (don't skip this part as I did or your brownies will be bumpy and uneven).
Bake the brownies for 30 to 33 minutes, or until the top is dull and a thin knife inserted into the center come out clean. Transfer the pan to a rack and cool the brownies to room temperature.
When the brownies are completely cool, turn out onto a rack, remove the parchment and invert onto a cutting board. Cut into 16 squares.