1. Melt the butter, sugar and cocoa in a large bowl in the microwave in 30 second increments, stirring between increments until the butter is fully melted. It will be gritty. Allow it to cool for a few minutes so that it isn’t too hot. You don’t want to cook the eggs when you add them in a couple steps.
2. Stir in the vanilla extract.
3. Add eggs one at a time, mixing well after each addition.
4. Add flour and mix until fully combined. Do not over mix.
5. Line an 8×8 baking pan with parchment paper. It’s best to have some hanging over the sides to help you lift the brownies out when they’re done.
6. Pour batter into pan.
7. Chop butterfinger bars (either by hand or with a food processor) and sprinkle them evenly on top of the brownie batter.
8. Bake at 325 degrees for 25-30 minutes, or until toothpick comes out with just a few moist crumbs.
9. Cool in pan until they can easily be lifted out without falling apart. Let cool completely before cutting into squares.