Preheat oven to 350 degrees F.
Lightly butter a 13 x 9-inch baking pan.
Sift together the flour and baking powder; set aside.
Beat in the vanilla.
Transfer half of the mixture to a second bowl.
Drop the remaining batter by tablespoonfuls onto the chocolate batter.
Spread carefully into an even layer.
Sprinkle with the remaining 1/2 cup chocolate chips and the walnuts.
Bake on the center rack for 30 minutes, or until the edges begin to pull away from the sides of the pan.