Preheat oven to 180 degrees C. Grease and flour a 20x30cm tin.
In a medium bowl cream butter with brown sugar and white sugar. Stir in the eggs and vanilla. Sift together the flour and bicarb soda, stir into the creamed mixture.
Spread the mixture evenly into the prepared pan. Sprinkle the chocolate chips over the top. Bake for 3 minutes. Remove from the oven and stir the chips into the batter to create a marbled effect. Place pan back into the oven and bake for an additional 15 to 20 minutes, until lightly browned. Cut into squares when cool.