Heat oven to 400°F. Coat 12-cup muffin pan with cooking spray.
Mix flour, sugar, baking powder, salt in bowl. Beat milk, prune puree, egg and vanilla in large bowl until blended.
Stir flour mixture into milk mixture. Stir in butter and chocolate chips. Spoon into muffin cups.
Bake in 400°F oven 17 minutes. Cool in pan 5 minutes. Serve warm.