Preheat oven to 350°F. Spray a 9x13 pan with non-stick spray or the butter & flour method.
In a medium bowl and using a hand-held mixer (or in a stand mixer with a paddle attachment) cream together the butter and sugar on medium-high for about 3 minutes.
Add in eggs, one at a time, and mix until fully incorporated.
Add in vanilla and mix well.
Add the salt, baking soda, and flour to the butter mixture and mix on medium until just combined. Add 1 ½ cups of the chopped Butterfinger (reserve ½ cup for sprinkling over the batter in the pan before baking.) Incorporate with the mixer on low.
Press the Butterfinger dough into the prepared 9x13 pan and press flat with a clean hand or a spatula. Sprinkle the remaining chopped Butterfinger over the dough.
Bake for 18-25 minutes or until the edges are golden brown. An inserted toothpick should be removed with crumbs, but not wet batter. IT IS IMPORTANT to not overbake these bars, they should be just set and will continue to bake a little out of the oven. They should be ooey-gooey.
Bars can be served warm or cold. Store in a sealed container on the counter for up to 3 days.