Preheat oven to 350°F. Beat peanut butter, brown sugar, corn syrup, butter, eggs and vanilla in large bowl with mixer at medium speed until smooth.
Reduce speed; beat in flour, baking powder and salt until well blended.
Stir in 1-1/2 cups of chocolate chips. Spread evenly in ungreased 15 X 10 X 1-inch baking pan.
Bake 25 minutes or until lightly browned. Cool completely on wire rack.
In small, heavy saucepan over low heat, stir remaining 1/2 cup chocolate chips until melted and smooth.
Drizzle over surface; cool before cutting.