Preheat oven to 325 degrees. Line baking sheet with parchment paper and set aside.
In medium pot over medium heat, melt butter. Remove from heat and stir in brown sugar and granulated sugar until well combined.
Add in egg, egg yolk and vanilla. Mix until lighter in color.
Add in the flour, baking soda and salt. Scrape the sides of the bowl and the bottom really well to ensure a smooth, well stirred batter.
Stir chocolate chips, chopped butterfinger and Reese's Pieces into warm batter. If batter seems too warm or sticky, let it sit at room temperature to cool 5-10 minutes. If some of the chocolate chips melt, that's ok!
Scoop dough onto prepared pans using a two tablespoon cookie scoop.
Bake 8-10 minutes, rotating sheets half way through baking. When you pull your cookies out of the oven, they will looked cooked around the edges and slightly undercooked in the center.
Leave the cookies on the hot baking pans for 5-7 minutes or until you can remove them without falling apart. Place onto cooling racks and cool to room temperature before storing in air tight containers, or enjoy warm.