Preheat oven to 350℉. Line baking sheet with parchment paper.
In a small glass bowl, microwave unsalted butter (½ cup, i.e., 1 stick) until melted. Pour into a large mixing bowl.
Add in granulated sugar (¼ cup), packed light brown sugar (½ cup), and peanut butter (½ cup) and mix for 10 seconds.
Add egg (1 large) and vanilla extract (1 ½ teaspoons) and mix on medium speed for 20 seconds.
Turn mixer to low speed, and add baking soda (½ teaspoon) and salt (¼ teaspoon). Mix for 10 seconds.
Half cup at a time, add all-purpose flour (1 ½ cups) and mix until flour is just incorporated (about 10-20 seconds). Do not overmix.
Scoop dough into (3 tablespoon sized) balls. Lightly flatten each ball before placing on cookie sheet.
Bake at 350℉ for 8 minutes. They will look underdone, but they will finish baking on the hot baking sheet after you take it out of the oven. (Of course, because ovens bake differently use your best judgement to keep them in a minute longer if they look severely underbaked.)
Immediately after removing from the oven, press one of the dipped BUTTERFINGER® Fun Size candy bars into the center of each cookie. Allow for the cookies to continue to cool on baking sheet for 5-10 minutes before removing. To harden chocolate, place in refrigerator for 15-30 minutes.