Instructions

• Preheat the oven to 325°F. Butter a 9-by-13-inch baking pan, line the bottom and long sides with parchment with a 2-inch overhang, then butter and flour the parchment.
• Whisk the flour and salt together. Melt the butter in a heatproof bowl set over, but not in, simmering water; remove from the heat and whisk in both sugars and then the cocoa powder.
• Whisk in the eggs one at a time until the mixture is glossy, mix in the vanilla, then stir in the flour mixture just until combined.
• Spread the batter evenly in the prepared pan and bake for about 30 minutes, until the top is crackly and a tester inserted in the center comes out clean.
• Cool in the pan on a wire rack for 30 minutes, lift out by the parchment, cool completely, and cut into 24 squares or rectangles.
• Beat the confectioners' sugar, meringue powder, and water on low speed for 7 to 8 minutes, until the royal icing is fluffy but dense.
• Pipe thin connected lines on square brownies to form spiderwebs.
• Pipe ribbons of icing back and forth on rectangular brownies to form mummy bandages, then add candy eyeballs.