Preheat oven to 350°F. Line two baking sheets with parchment paper.
Beat butter and sugar in a large bowl until creamy and light, using a hand mixer or stand mixer fitted with a paddle attachment.
1 cup salted butter, 1 cup brown sugar, ¾ cup granulated sugar
Add peanut butter and vanilla extract. Beat well, stopping to scrape the bottom and sides of the bowl.
1 cup creamy peanut butter, 2 teaspoons vanilla extract
Add the eggs and beat again until combined.
2 large eggs
Add flour, baking soda, cornstarch, and salt. Mix just until combined. Stir in 1½ cups of chopped butterfingers into the cookie dough, reserving the remaining chopped candy bars for the tops of the cookies.
3 cups all-purpose flour, 1 teaspoon baking soda, ¼ teaspoon salt, 1 teaspoon cornstarch, 2½ cups chopped butterfinger candy bars
Scoop large balls of cookie dough onto a baking sheet lined with parchment paper, spacing them a couple of inches apart so they have room to spread. Top each cookie with additional crushed butterfingers, pressing them in so they don't fall off while baking.
Bake in a preheated oven at 350°F for 10–12 minutes until cookies are set around the edges and slightly underbaked through the middles. Cool on the pan for 10–15 minutes before transferring to a wire rack to cool completely.