To get that perfect surface zigzag:
Pour your favorite brownie batter into a greased pan. Fill a piping bag with cream cheese mix (for a 9″ square pan: 8oz box of cream cheese, an egg, sugar and perhaps a flavoring or extract). Pipe thick lines of cream cheese filling across the length of the pan. Drag a knife through each line of cream cheese, alternating direction with each stroke. Voila!