HEAT oven to 425°F.
USE 1 Tbsp. butter to grease insides of 4 (150-mL) ramekins; place on baking sheet. Microwave chocolate and remaining butter in large microwaveable bowl on HIGH 1 min. or until butter is melted. Whisk until chocolate is completely melted. Stir in sugar. Blend in whole eggs and egg yolks; stir in flour.
POUR half the batter evenly into prepared ramekins; top with peanut butter and 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.
serving size = 1/2 molten cake (63 g)