Preheat your oven to 350°F (175°C). This ensures the brownies bake evenly and thoroughly.
Line a 9x13 inch baking dish with parchment paper, leaving some overhang on the sides for easy removal later.
In a medium saucepan over low heat, combine the chopped unsweetened chocolate and butter. Stir occasionally until melted and smooth. Remove from heat and allow to cool slightly.
In a large mixing bowl, combine the sugar, eggs, vanilla extract, and salt. Whisk until the mixture is smooth and well-combined. Gradually add the melted chocolate mixture, mixing until fully incorporated.
Gently fold in the all-purpose flour using a spatula, being careful not to overmix. The batter should be thick and fudgy.
Fold in the chopped Butterfinger candy bars, making sure they are evenly distributed throughout the brownie batter.
Pour the brownie batter into the prepared baking dish and spread it evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before cutting into squares.
