Preheat oven to 350 degrees F. Line a 13x9 inch metal pan with nonstick foil (or grease it).
Prepare cream cheese mixture first. Beat cream cheese, sour cream, sugar and vanilla in a small mixing bowl. Reduce speed to low and beat in the egg. Set aside.
Brownie Batter: Melt the chocolate in the microwave using high power and stirring every 30 seconds. Set aside to cool slightly.
With an electric mixer, beat softened butter and sugar until creamy. Beat in the eggs, one at a time. Beat in salt, vanilla and almond extract (if using). Beat in the melted chocolate.
By hand, stir in the flour until it is incorporated, then stir in the chopped walnuts and chocolate chips.
Spread chocolate mixture evenly in pan. Drop spoonfuls of cheesecake mixture over brownie batter, then very lightly drag a knife through to make the batter look marbled and blotchy – I don’t drag it too much because I like big clumps of cream cheese mixture.
Bake at 350 F. on center rack for 35-45 minutes.
Let cool completely, then chill for a few hours. Lift from pan and cut into 24 squares.