In a large bowl, combine flour, brown sugar, baking powder, salt, baking soda, 1/2 cup butter or margarine, 1 teaspoon vanilla and egg yolks. Mix on low speed with an electric mixer until crumbly.
Press firmly into bottom of 13 x 9 inch pan. Bake 12- 15 minutes until golden brown.
Immediately sprinkle marshmallows over top and return to oven for 2 - 2 1/2 minutes. Let cool while preparing topping.
To Make Topping: In a large saucepan over low heat combine corn syrup, 1/4 cup butter or margarine, 2 teaspoons vanilla and peanut butter chips until smoothly melted, stirring constantly. Remove from heat and stir in cereal and nuts. Immediately spoon over marshmallows, spreading to cover. Refrigerate until firm. Cut into 36 bars.