Preheat oven to 350° F. Grease 15 x 10 baking pan. In a large bowl with electric mixer, cream butter and peanut butter. Blend in brown sugar, beat in eggs one at a time until creamy. Combine flour, baking powder and salt in a small bowl, set aside. Combine milk and vanilla in measuring cup. Add dry ingredients and milk alternately to creamed mixture. Mix at low speed, scraping sides of bowl frequently, beat until blended. Spread batter into prepared pan. Bake for 18 to 20 minutes. Cool. For frosting: cream butter and peanut butter in large bowl at medium speed with electric mixer. Add powdered sugar and milk; beat until fluffy. Spread on cookie base. For drizzle: melt chocolate chips on very low heat in small saucepan. Drizzle chocolate from end of spoon, back and forth over frosting. Cut into 2-inch squares. Refrigerate 15 to 20 minutes until chocolate is firm.