Brown & white sugar
Beat together the butter and sugar on medium speed, then add the peanut butter, egg yolk, milk, and vanilla extract. Then, mix in the dry ingredients.
Roll the peanut butter dough balls into 1-inch balls and place them on prepared cookie sheets lined with parchment paper.
Roll the dough through the whisked egg whites, then through the finely chopped peanuts until completely coated.
Bake the cookies evenly apart with enough space to spread slightly until slightly golden brown, then use a ½ teaspoon measure to press a well into the center of each cookie.
Use a round object like a drinking glass or cookie cutter to "scoot" the cookies. This helps to patch any cracks that may have appeared during the indentation process.
Fill the bottom of the well with a ½ teaspoon of melted peanut butter, then top with your favorite jam. Cool completely on the pan, then enjoy!