Preheat oven to 350 degrees and spray two 2 cup ramekins with cooking spray and the bottoms lined with parchment paper.
Heat the milk in a microwave safe bowl until hot, about 45-60 seconds. Remove and add chocolate chips. Let sit for 1 minute then stir to melt.
In a stand mixer, mix the flour, cocoa powder, sugar, salt and baking soda to combine. Slowly add the eggs, sour cream, oil and melted chocolate mixture until combined. Divide batter between the two prepared ramekins. Bake for about 30 minutes, or until baked through. Remove and let cool. Remove cakes from ramekins and split horizontally.
To prepare frosting, beat butter until creamy. Slowly add powdered sugar and cocoa until thick and spreadable. Spread a thin layer over each of the split cakes. Top each frosted layer with crushed Butterfinger bars. Stack cakes on top of each other onto your desired cake plate. Frost entire cake and top with more crushed Butterfinger bars.