Preheat the oven to 350°F. Line a 9 × 13-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal. Lightly grease the parchment paper to prevent sticking.

Combine the cubed butter and chopped bittersweet chocolate in a microwave-safe bowl. Heat in 20–30-second intervals, stirring between each interval, until the mixture is completely smooth. Set aside for about 5 minutes to cool slightly.

In a large mixing bowl, whisk together the eggs, brown sugar, and granulated sugar until well combined. Slowly pour in the melted chocolate mixture, whisking continuously. Stir in the vanilla extract and espresso powder, if using.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Gently fold the dry ingredients into the chocolate mixture using a silicone spatula. Mix just until no dry streaks remain to keep the brownies tender and fudgy.

Pour the batter into the prepared pan and spread it evenly. Bake for 25–32 minutes, or until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs. Avoid overbaking, as the brownies will continue to firm up as they cool.

Let the brownies cool for about 10 minutes, then scatter the chopped chocolate bars, mini M&M’s, Reese’s Pieces, candy corn, candy eyeballs, and Halloween sprinkles over the top. Gently press the toppings into the warm surface so they adhere. If the brownies have cooled too much for the toppings to stick, warm them briefly in the oven for 1–2 minutes first.

Allow the brownies to cool completely in the pan, about 1 hour. Use the parchment overhang to lift them onto a cutting board. Slice into 24 squares with a sharp knife, wiping the blade between cuts for clean edges. Serve at room temperature.