Preheat oven to 425°F Spray six jumbo muffin cups with non-stick cooking spray. Microwave chocolate and butter in large microwaveable bowl on HIGH 1 minute or until butter is melted.
Stir with wire whisk until chocolate is completely melted. Stir in sugar until well blended. Blend in eggs and egg yolks with wire whisk. Stir in flour and nuts. Divide the batter amongst the cups. Bake 13 to 14 minutes or until sides are firm but centers are soft.
Let stand 1 minute Carefully run small knife around cakes to loosen (if necessary). Invert cakes onto dessert dishes. Serve immediately, topped with cool whip or powdered sugar.
TO MAKE BATTER A DAY AHEAD: Prepare the batter as directed and pour into greased jumbo muffin cups. Cover with plastic wrap. Refrigerate up to 24 hours. Let stand at room temperature 1 hour before baking as directed.