1 stick (4 ounces) unsalted butter, cut into ½-inch pieces
4 ounces semisweet or bittersweet chocolate (up to 64 percent cacao), finely chopped
2 ounces unsweetened chocolate, finely chopped
1 cup (7 ounces) sugar
2 large eggs, at room temperature
1 ½ teaspoons pure vanilla extract
½ cup (2 ½ ounces) unbleached all-purpose flour
Pinch of salt
½ cup (about 3 ¾ ounces) chocolate chips or chocolate chunks
Cream Cheese Layer:
8 ounces (227 grams) cream cheese, at room temperature
1/3 cup (65 grams) granulated white sugar
1 teaspoon pure vanilla extract
1 large egg
Preheat oven to 350F and position an oven rack in the center. Line the pan with foil or parchment paper across the bottom and up two of the sides, then lightly coat with unflavored oil or high-heat canola-oil spray.
Bring 2 inches of water to a boil in the bottom of a double boiler. Place the butter, semisweet chocolate, and unsweetened chocolate in the top of the double boiler (off the heat). Turn off the heat, then set the butter and chocolate over the steaming water. Stir occasionally with the spatula until the chocolate is melted and the mixture is smooth.
Remove the chocolate mixture from the heat and whisk in the sugar. Whisk in the eggs, one at a time, stirring well to incorporate each before adding the next. Stir in the vanilla extract. Whisk in the flour and salt. Continue to stir until the mixture changes from dull and broken-looking to smooth and shiny, about 1 minute. Whisk in the chocolate chips and chopped nuts, if using.
Scrape the batter into the prepared pan and spread evenly.
Then, in the bowl of your food processor (or with a hand mixer), process the cream cheese until smooth. Add the sugar, vanilla, and egg and process just until creamy and smooth. Spread the cream cheese filling evenly over the brownie layer. Spoon small dollops of the reserved brownie batter evenly on top of the cream cheese filling. Then with a table knife or wooden skewer, swirl the two batters without mixing them.
Bake for 35 to 40 minutes, until a skewer inserted into the centre of the brownies comes out with a few moist crumbs clinging to it (do not over-bake). Transfer to a rack and cool completely.
Run a thin knife or flexible spatula around the edges of the pan to loosen the brownies. To remove the brownies from the pan, grasp the foil or parchment paper extending up the sides and gently pull upward. Set the brownies on a cutting surface and use a chef’s knife to cut into 16 equal pieces. Since these are dense, it’s a good idea to keep a hot, wrung-out towel nearby so you can wipe the knife clean between slices.