In a large bowl, cream the butter and brown sugar; beat until light and fluffy.
Beat in the eggs, vanilla, flour and baking powder until well mixed.
Melt 1 cup of the chocolate chips in the microwave or over low heat on the stove.
Place half of the batter into a medium-sized bowl and stir in the melted chocolate.
Spread evenly in a greased 9x13 baking pan.
Drop tablespoons of the remaining batter over the chocolate batter and drag the blade of a butter knife through the batter to create zebra stripes. Sprinkle with the remaining chocolate chips and walnuts.