Here is the recipe as found on the original website. You can also replace the first section of ingredients with a box-brownie mix to save time. I also used a 9x13 in glass baking dish, which worked fine as well.
One 9-inch (23cm) square pan
6 tablespoons (85g) unsalted butter, cut into pieces
4 ounces (115g) bittersweet or semisweet chocolate, chopped
2/3 cup (130g) sugar
2 large eggs, at room temperature
1/2 cup (70g) flour
1 tablespoon unsweetened cocoa powder
1/8 teaspoon salt
1 teaspoon vanilla extract
1/2 cup (80g) chocolate chips
8 ounces (200g) cream cheese, at room temperature
1 large egg yolk
5 tablespoons (75g) sugar
1/8 teaspoon vanilla extract
Optional: Several tablespoons of Nutella
1. Line a 9-inch (23cm) square pan with foil, making sure it goes up all four sides. Use two sheets if necessary. Mist with non-stick spray or grease lightly. (I didn't use the foil, but I think this might make it much easier if you're planning on cutting the brownies out to serve on a platter)
2. Preheat oven to 350 degrees (180C).
3. In a medium saucepan, melt the butter and chocolate over low heat, stirring until smooth. Remove from heat and beat in the 2/3 cup (130g) sugar, then the eggs.
4. Mix in the flour, cocoa powder and salt, then the vanilla and chocolate chips. Spread evenly in the prepared pan.
5. In a separate bowl, beat together the cream cheese, the yolk, 5 tablespoons (75g) of sugar, and vanilla until smooth.
6. Distribute the cream cheese mixture in eight dollops across the top of the brownie mixture, then take a dull knife or spatula and swirl the cream cheese mixture with the chocolate batter. (Before swirling the mixture, I just drizzled a few spoonfuls of Nutella and then swirled)
Let cool, then lift out the foil and peel it away. Cut the brownies into squares if desired!