1. Preheat oven to 350°F. Line an 8-inch square baking pan with foil, leaving a 1-inch overhang on 2 sides. Mist foil with cooking spray.
2. Make swirl: Using a mixer on medium speed, beat cream cheese and butter until creamy. Add egg; beat until smooth. Beat in sugar, flour and vanilla until smooth. Set aside.
3. Make brownies: In a double boiler, melt butter with chocolate, stirring often. Remove from heat; stir in sugar, vanilla and salt. Transfer to a bowl; cool slightly.
4. Using a mixer on medium-low speed, beat eggs into chocolate mixture 1 at a time. Sprinkle flour over mixture and gently stir in.
5. Spread 1/2 of brownie batter in prepared pan. Carefully spread swirl mixture over brownie batter. Spoon remaining brownie batter on top. Use tip of a knife to gently swirl through both batters to form a marble pattern.
6. Bake until a toothpick inserted into center comes out clean, 40 to 45 minutes. Let cool on a wire rack for at least 1 hour. Remove brownies from pan to cutting board using foil to lift them out. Peel off foil and cut into squares.