Preheat oven to 375°F.
Grease 9 x13 pan and set aside.
In a mixing bowl, cream together the butter, sugars, and vanilla until fluffy.
Add the egg and mix.
Blend in the flour, baking soda, and salt. Mix well.
Spread evenly in the pan. Sprinkle with the chocolate chips.
Bake for 3 minutes.
Remove from the oven and run a knife through the brownies in a zigzag pattern to make a nice marble swirl.
Return to the oven and bake for 13-15 minutes.
Let cool and then cut into squares.