Using a stand or a hand mixer, beat the butter and sugar for about 90 seconds.
Add egg and egg yolk and beat for 1 minute.
Mix the dry ingredients in a separate bowl.
Combine the wet and dry ingredients and mix until flour is baking streaking through, being careful not to over mix.
Mix in the chopped Butterfingers. Candy bar should be roughly chopped in about ¼" to ½" pieces. Smaller pieces are ok.
Roll into 1 ½ inch balls with your hands and place on a large parchment paper lined cookie sheet.
Bake for 10 minutes, until the edges start to brown. Remove from oven.
Let cool for 5 minutes on the cookie sheet, then remove and place on a rack to finish cooling.
Be careful not to over mix the dough or the cookies will be less light and tender;
Using parchment paper keeps any of the melted Butterfinger from sticking to your baking sheet. A silicone liner (affiliate link) would also would great.
You can use a cookie scoop (affiliate link) if you want to cookies to come out the same size.
To freeze: Once the cookies have cooled completely, place them on a baking sheet directly in the freezer for 3 hours. Store them in a freezer bag with layers of wax or parchment paper in between the cookies to prevent them sticking together.
To thaw frozen cookies, let them stand at room temperature until they are completely thawed (3-6 hours is the most common time). You can also reheat them in a 350 degree oven until warmed through and lightly browned to give it more of a "fresh baked" taste.