GREASE an 8-inch square baking pan; PREHEAT oven to 350F.
In a small saucepan, STIR butter and peanut butter over low heat; when melted and smooth, REMOVE from heat.
In a large mixing bowl, BEAT eggs at high speed two minutes, until pale yellow.
ADD sugar; BEAT two minutes longer; ADD melted butter mixture, cocoa and vanilla; BEAT again at high speed until smooth.
ADD flour and baking powder; BEAT three minutes longer, batter will be very thick.
SCRAPE batter into prepared pan; BAKE 25 minutes.
REMOVE baked brownies from oven; REFRIGERATE at least two hours before serving.
Per Serving (excluding unknown items): 268 Calories; 11g Fat (37.4% calories from fat); 5g Protein; 37g Carbohydrate; 2g Dietary Fiber; 69mg Cholesterol; 148mg Sodium. Exchanges: 1/2 Grain(Starch); 1/2 Lean Meat; 2 Fat; 2 Other Carbohydrates.