In a large bowl, cream together the butter and sugar until smooth.
Mix in the egg yolks (save the whites) and vanilla.
In a separate bowl, whisk together the flour and salt.
Add the dry ingredients to the wet and mix until well combined, scraping the sides of the bowl as needed.
Refrigerate the dough for 1 hour.
Preheat the oven to 350º Fahrenheit. Line baking sheets with nonstick baking mats.
Place the egg whites in a small bowl and use the fork to whisk lightly.
Place the chopped walnuts in another bowl.
Roll out the dough into 1" balls. Dip the balls into the egg whites, then roll in the chopped nuts.
Place on a baking sheet and bake for 5 minutes. Remove the tray from the oven and press your thumb into the top of each cookie to make a well.
Return the pan to the oven and bake another 5 minutes.
Move the cookies to wire racks to cool completely.
Once the cookies are cool, fill the centers of the cookies with vanilla buttercream. Do this by snipping of the corner of a zippered bag and filling with buttercream. Squeeze the frosting through the tip to fill the cookie. You can also use a cake decorating bag.
Then press a chocolate chunk or chocolate candy into the center of the frosting.
The calories shown are based on the recipe making 36 cookies, with 1 serving being 1 cookie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**