Preheat oven to 180 degrees C. Grease a 20x30cm baking pan.
To make base, cream 85g copha and 1/2 cup peanut butter in large bowl at medium speed in an electric mixer. 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 1/4 cup milk and 1 teaspoon vanilla in measuring cup. Add dry ingredients and milk alternately to creamed mixture. Mix at low speed. Scrape the sides of the bowl frequently and beat until blended.
Spread batter into prepared pan and bake for 18 to 20 minutes. Let cool.
For icing: cream 65g copha and 2/3 cup peanut butter in large bowl at medium speed of electric mixer. Add 4 cups of icing sugar and 1/2 cup milk. Beat until fluffy. Spread on cooled base. For drizzle melt chocolate pieces on very low heat in small saucepan. Drizzle chocolate from end of spoon back and forth over icing. Cut into 5cm squares. Refrigerate 15 to 20 minutes until chocolate is firm.