Heat oven to 350°. In large bowl, combine flour, brown sugar, baking powder, salt, baking soda, 1/2 cup softened butter, 1 teaspoon vanilla, and egg yolks until crumbly. Press firmly in bottom of ungreased 9X13X2-inch baking pan. Bake for 12 to 15 minutes or until lightly browned. Remove from oven and immediately sprinkle with marshmallows. Return to oven and bake an additional 1 or 2 minutes until marshmallows just begin to puff. Cool while preparing topping.
In a large saucepan, combine corn syrup, 1/4 cup butter, vanilla and peanut butter chips. Over low heat melt just until chips are melted and mixture is smooth, stirring constantly. Remove from heat and stir in peanuts. Immediately spoon warm topping evenly over marshmallows; dipping your fingers in water to prevent stickiness, spread to cover. Refrigerate until firm; 1 hour and cut into bars dipping the knife into hot water occasionally.