Preheat oven to 350°F. Crush 10 of the candy bars, leave 24 whole fun-size candy bars.
In a large bowl of a stand mixer fitting with a paddle, combine peanut butter, butter, brown sugar, milk and vanilla on medium speed until just blended. Add the egg to the peanut butter mixture and continue to beat until blended.
In a medium bowl, combine flour, baking soda and salt.
With the mixer on low, slowly add the dry ingredients to the peanut butter mixture. Blend until the flour mixture is incorporated. Fold in the crushed Butterfinger pieces until evenly distributed throughout the dough.
Roll dough, about 2 tablespoons each, into balls and place on lined cookie sheet 2 inches apart.
Bake for 10 - 15 minutes, until cookies begin to brown around the edges.
Immediately after removing the cookies from the oven, place a whole Butterfinger Fun-Size bar into the center of each cookie and press down gently. Allow to cool until the chocolate has set. Decorate as desired.