Cream butter or margarine, sugar and vanilla. Add eggs; beat well. Combine flour and baking soda; add alternately with chocolate syrup to creamed mixture. Stir in peanut butter chips. Pour batter into greased 9 x 13 pan; bake at 180ºC for 30 to 35 minutes. Cool; frost with Peanut Butter Brownie Frosting.
Frosting: Combine sugar, evaporated milk and butter in a small saucepan. Stir over medium heat until mixture comes to full boil; remove from heat. Quickly stir in 1 cup peanut butter chips until melted; add vanilla. Beat to spreading consistency; frost brownies.