Preheat the oven to 350°F. Line a 9x9-inch baking pan with two pieces of parchment paper, creasing the paper over the edges to keep it in place. If you're not using a nonstick pan, grease any exposed corners.

Add the butter and chocolate to a medium microwave-safe bowl. Microwave in 30-second increments, stirring between each, until completely melted and smooth. This takes approximately 2 minutes, depending on your microwave.

In a small bowl, stir together the granulated sugar and salt. Add the mixture to the melted chocolate and stir until combined.

Whisk in the eggs, followed by the flour, and finally the vanilla extract. Stir until everything is incorporated.

Pour the batter into the prepared baking pan and spread evenly. Bake for 35–37 minutes.

While the brownies bake, unwrap the mini candy bars and gather the candy corn, candy pumpkins, and Halloween sprinkles.

As soon as the brownies come out of the oven, carefully arrange the candy bars and other Halloween candy over the hot surface. Gently press candy-coated chocolates like M&M's or Smarties into the top if needed. The heat will soften the chocolate bars and help them stick to the brownies as they cool.

Allow the brownies to cool for at least 20 minutes before cutting. The chocolate bars may still be soft. If you prefer the candy completely set, refrigerate or freeze the brownies after they've cooled on the counter for at least 20 minutes.