In a medium-sized bowl or a food processor, add the Oreo cookies and blend until the texture of coarse meal or crumbs. You can also put the cookie or cookie pieces into a large zip lock bag and use a rolling pin to crush. Add the melted butter too the crushed cookies and blend until well combine.
Place the ground crumb mixture into a 9x13 inch pan and press onto the bottom. Try to make the crust about 1/8 inch evenly on bottom and sides. I use the back of a measuring cup to press the Oreo mixture onto the pan.
Place crust in refrigerator to help set it while you work on the next steps.
In a bowl, combine flour, baking powder and salt. Set aside.
In large mixing bowl, cream butter, sugar and vanilla together. Add 1 egg and 1 egg yolk and combine well.
Slowly add the flour mixture to the butter mixture and combine well.
Remove crust from the refrigerator, and drop the cookie dough onto the crust. The crust is not set, so it will be very loose. A good way to do this is to use a cookie scoop and drop even amounts of dough around the entire 9x13 pan and then lightly press with a metal spatula so that it is spread fairly evenly.
Next, place the baking squares onto the cookie mixture leaving 1/2 inch between each square. It should look like a checker board pattern.
In another bowl, combine brown sugar, crushed walnuts and egg whites. Drop mixture onto the top of the baking pan and spread evenly. don't worry if it doesn't completely cover all the cookie, you can have some cookie poking through.
Place in 325 degree preheated oven and bake for 45 minutes.
Remove from oven and cool completely before cutting into squares and serving.