HEAT oven to 350ºF. Microwave unsweetened chocolate and butter in large microwaveable bowl on MEDIUM 3 min.; stir until completely melted. Add eggs; mix well. Stir in sugar and flour.
SPREAD into greased 13x9-inch pan.
BAKE 30 min. Immediately top with spoonfuls of peanut butter and semi-sweet chocolate; swirl with knife. Cool.