Cut into 16 squares.

Ingredients:

5 tablespoons (70g) unsalted butter, cut into 5 pieces
112g (4 ounces) semisweet chocolate, coarsely chopped
56g (2 ounces) bittersweet chocolate, coarsely chopped
¾ cup (150g) sugar
2 large eggs
1 teaspoon pure vanilla extract
½ teaspoon instant espresso powder (optional, I used it)
¼ - ½ teaspoon salt (according to taste, I used ¼)
1/3 cup (47g) all purpose flour
1 cup (115g) chopped walnuts

Directions:
Center a rack in the oven and preheat the oven to 160ºC/325ºF. Line a 20cm (8-inch)* square baking pan with foil, butter the foil and place the pan on a baking sheet.
Set a heatproof bowl over a saucepan of simmering water. Put the butter in the bowl, top with the chocolates and stir occasionally until the ingredients are just melted - you don’t want them to get to shot 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 before gently stirring tin the espresso, if you’re using it, salt and flour; stir only until incorporated. Switch to a rubber spatula and fold in the chopped walnuts.

Scrape the batter into the pan and smooth the top with the spatula.

Bake the brownies for 30 to 33 minutes or until the top is dull and a thin knife inserted into the center comes out clean. Transfer the pan to a rack and cool the brownies for room temperature.

When the brownies are completely cool, turn out onto a rack, peel away the foil and invert onto a cutting board – if you do this, the cracked top might be ruined. I preferred to use longer foil pieces, forming “handles” in the baking pan – then I just lifted the whole thing, placed onto a cutting board and cut the brownies, removing them carefully from the foil.