In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolks and vanilla. Combine the flour, baking powder, salt and baking soda; gradually add to the creamed mixture until mixture resembles coarse crumbs (do not overmix).
Press into a greased 13-in. x 9-in. baking pan. Bake at 350° for 12-14 minutes or until golden brown. Immediately sprinkle with marshmallows; bake 2-3 minutes longer or until marshmallows are puffed. Cool on a wire rack.
For topping, in a large saucepan, combine the corn syrup, butter and peanut butter chips. Cook and stir over medium heat until chips are melted and mixture is smooth. Remove from the heat; stir in the vanilla, cereal, nuts and M&M's. Spread over crust. Cool before cutting.