HEAT oven to 425°F.
BUTTER 4 (3/4-cup) custard cups or soufflé dishes. Place on baking sheet. Microwave chocolate and butter in large microwaveable bowl on HIGH 1 min. or until butter is melted. Stir with whisk until chocolate is completely melted. Add sugar; mix well. Blend in whole eggs and egg yolks. Stir in flour.
POUR half the batter evenly into prepared cups. Spoon 1 Tbsp. peanut butter into batter in centre of each. Cover with remaining batter.
BAKE 14 min. or until sides are firm but centres are still soft. Let stand 1 min. Carefully run small knife around edges of cakes to loosen. Invert cakes onto dessert plates. Cut in half to serve.