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 the salt, baking soda, and flour to the butter mixture and mix on medium until just combined. Add 1 1/2 cups of the chopped Butterfinger (reserve 1/2 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.