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.