Preheat oven to 350 degrees F.

Add butter, brown sugar and granulated sugar to a mixing bowl and cream together until smooth and light. Add the vanilla and eggs and mix well.

Add flour, baking soda and salt and mix on low speed until just combined. Stir in chopped Butterfinger bits.

Smooth mixture into a lightly greased 9×13 inch baking pan. Bake for 24-26 minutes, until center of bars are set, and no longer glossy looking.

Remove from oven and cool completely before frosting.

Using an electric mixer, beat the peanut butter and butter on medium-speed until completely smooth. Add vanilla and mix.

Reduce speed to low and add in the remaining ingredients, beating until fully incorporated. Increase speed to medium-high and beat the frosting for 1-2 minutes, or until light and fluffy.