Preheat your oven to 350°F. Grease an 8x8 baking pan and line it with parchment paper, if desired. Set it aside.
In a large bowl, whisk together the melted butter, vegetable oil, granulated sugar, eggs, egg yolk, and vanilla extract. Whisk for around a full minute until mixture starts to look a little frothy.
Add in the flour, cocoa powder, baking powder, salt, and espresso powder and whisk until everything is combined and the batter is smooth.
Fold in the cup of leftover Halloween candy (bars chopped into pieces).
Pour the brownie batter into the prepared pan and spread it out evenly. Pop into the preheated oven and bake for 30-32 minutes. Once out of the oven, immediately and carefully press more Halloween candy into the tops of the brownies, if desired. Allow the brownies to cool completely before slicing.