Preheat oven to 350 degrees F (180 degrees C).
In a large mixing bowl, beat Butter (1 cup), Caster Sugar (1 cup), and Vanilla Extract (1 teaspoon) medium speed of an electric mixer until well combined. Beat in Egg Yolk and All-Purpose Flour (2 cup). Stir in Walnut (1 cup).
Spread into a 10-inch by 15-inch Pyrex by 2-inch baking dish that has been sprayed with Nonstick Cooking Spray (to taste).
Now, beat the white of the Egg (1) until frothy. Spread beaten egg white over mixture in baking dish. Sprinkle the remaining Walnuts (1/2 cup) [Chopped] over the top.
Bake for about 20 to 25 minutes, or until set and lightly browned. A toothpick inserted into the center should come out clean.
Immediately sprinkle with Semi-Sweet Chocolate Chips (1/2 cup) and Peanut Butter Chips (1/2 cup). Let stand 5 minutes.
Spread melted chips to form marbled frosting. Cool completely and cut into bars. Enjoy!