Position a rack in the center of the oven and preheat to 350 degrees F. Line a 13-by-9-inch baking pan with aluminum foil so that the foil extends 2 inches beyond the long sides of the pan. Lightly butter the bottom and sides of the foil-lined pan.

Melt the chocolates according to the melting instructions in the Chocolate Key. Cool the chocolate for 10 to 15 minutes, until tepid. In a large bowl, using a hand-held electric mixer set at medium speed, beat together the cream cheese and 1/3 cup of the sugar until smooth. Beat in one of the eggs and 1 teaspoon of the vanilla until combined. In another large bowl, using a hand-held electric mixer set at medium speed, beat the butter and remaining 1 2/3 cups of sugar until combined.

One at a time, beat in the remaining 3 eggs, beating well after each addition. Beat in the melted chocolate and the remaining 2 teaspoons of vanilla extract. On low speed, beat in the flour and salt, just until combined.

Scrape all but 1 cup of the chocolate batter into the prepared pan and smooth the top with a rubber spatula. Spread the cream cheese mixture evenly over the batter. Spoon the reserved chocolate batter over the cream cheese mixture. Using a table knife, pull it through the layers of batter with a slight lifting motion, in a zigzag pattern to create a marbleized pattern. Bake for 30 to 40 minutes, or until a cake tester or toothpick inserted 2 inches away from the center comes out with a few moist crumbs clinging to it.

Cool the brownies in the pan set on a wire rack. When the brownies are completely cooled, using the two ends of the foil as handles, lift the brownies out of the pan. Cut into squares.