Preheat the oven to 180 C / Gas 4. Grease a 20x30cm baking tin.
In a large bowl, cream together butter or margarine, dark brown soft sugar and 100g caster sugar. Beat in the 3 eggs one at a time, then stir in the vanilla. Combine the flour, bicarb and salt; blend into the sugar mixture. Stir in chocolate chips, and set aside.
In a separate bowl, mix together the cream cheese and 4 tablespoons caster sugar. Mix in the 2 remaining eggs.
Spread 1/2 of chocolate chip mixture in bottom of the prepared tin. Pour cream cheese mixture on top of chocolate chip mixture. Sprinkle with nuts. Drop pieces of the remaining chocolate chip mixture over filling. Don't worry if there are gaps; the mixture will spread.
Bake for 45 minutes in preheated oven, or until lightly browned on the top. Let the brownies cool in the tin before cutting into squares.
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