Preheat oven to 350 degrees.
Use a stick of butter to grease a 9x13 baking pan.
Melt the stick of butter in the microwave.
In a large bowl, stir together the cake mix, butter, eggs and vanilla. Add chocolate chips last, stir until combined.
Spread mixture evenly in bottom of baking pan.
Bake approximately 20 minutes at 350. Let cool, cut into bars and serve!