Position an oven rack in the center of the oven and preheat to 350 degrees F. Generously coat a 9-by-13-inch baking dish with nonstick spray and line with parchment with a 2-inch overhang on the long sides. Coat the parchment with the spray as well.
Whisk together the flour and cocoa in small bowl. Set aside.
Fill a medium saucepan with 1 inch of water. Bring to a boil, then reduce the heat to low. Combine the butter and 8 ounces of the chocolate in a medium heatproof bowl. Set the bowl over the saucepan and heat, stirring, until melted and smooth, about 6 minutes. Set aside. Reserve the saucepan and water.
Vigorously whisk together the brown sugar, granulated sugar, vanilla, salt and eggs in a large bowl until the mixture is light and foamy. Stir in the chocolate-butter mixture. Fold in the flour mixture. Pour the batter into the prepared baking dish and spread evenly. Bake until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Let cool completely.
Meanwhile, bring the water in the reserved saucepan back to a boil, then reduce the heat to low. Set another medium heatproof bowl over the saucepan, add the remaining bittersweet chocolate and heat, stirring, until melted, about 6 minutes.
Use the parchment overhang to remove the cooled brownie from the baking dish and transfer to an upside down rimmed baking sheet. Trim the parchment so that it is flush with the brownie edge and slightly hidden.
Transfer 1/4 cup of the melted chocolate to a small bowl and keep warm. Pour the remaining chocolate over the brownie and spread to evenly cover. Cool until set.
Cut off the uncoated ends of the biscuit sticks (discard or eat) so only the chocolate-covered parts remain. Cut each chocolate-covered part into 8 equal pieces. In pairs, dip one end of each small piece into the reserved melted chocolate and lay the pairs on a baking sheet so that they overlap slightly in the shape of a V. Repeat with remaining biscuit pieces until you have 32 total Vs (these will be the legs for your spiders). Refrigerate until set.
Trim the bottoms off the malted milk balls so they lay flat. Set aside.
Microwave the marshmallows in a medium microwave-safe bowl, stirring once or twice, until melted, about 30 seconds.
Working near the brownie, dip the tips of your fingers in the melted marshmallow. Lightly press your fingers together then pull them apart, stretching out webbed bands of marshmallow. Carefully place on the top and sides of the brownie to create a spider web pattern.
To make spiders on top of the marshmallow web: arrange the trimmed malted milk balls on the web cut-side down. Place a chocolate-coated candy in front of each malted milk ball, pressing gently to adhere to the chocolate. Arrange 4 sets of legs on each side of each spider body, pressing gently to adhere. Lightly dust the spiders with red luster dust.
Cut the brownie into squares and serve.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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