Preheat oven to 350 degrees F.
Preparation of the Baking Pan - Grease the bottom and sides of the metal baking pan (8-inches square). Lay a large square piece of parchment paper in the pan so that there is about 2 inches of paper extending over the edges to use as a “sling” when removing the brownies from the pan.
Make a crease in the parchment paper in each corner so the paper doesn't bunch up.
Lightly press the paper to the bottom and sides of the pan so it sticks to the butter. Grease the paper that’s inside of the pan with butter. Set aside.
Cream the Butter, Sugar and Eggs - To a large mixing bowl, add the butter, sugar and eggs. Beat on high with a mixer until light and creamy. Set aside for a moment.
Melt the Chocolate - In a small microwave-safe bowl, add the chocolate chips. In a microwave, heat the chocolate chips on the low setting for two 30 second intervals while stirring in between the intervals.
Just before the chocolate chips are completely melted (about two 30-second intervals) stir until smooth and creamy.Let the melted chocolate sit for a few minutes so that it cools just a bit before adding it to the creamed butter mixture. If the chocolate is too hot when added to the mixture, it may curdle the eggs.
While the chocolate is cooling, to the creamed butter mixture, add the milk. After the melted chocolate has cooled for a few minutes add it to the mixing bowl.
To the bowl, add baking powder, salt and flour. Mix until combined.
Pour half of the batter into the parchment paper lined-pan. Using a soft spatula, smooth the batter so that it's level. Add the pieces of butterfinger bars evenly on top of the batter.
Top with the remaining batter. Use a soft spatula to smooth the batter so that it's level.
Bake in a preheated oven until the sides of the brownies start to pull away from the pan and a toothpick inserted in the middle comes out clean (about 26 to 29 minutes). Remove from the oven onto a cooling rack and let completely cool (while the brownies are still in the pan).