Scale

Chewy brownies:

Spider webs:

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Prep. Preheat the oven to 325°F (160°C / 150°C fan). Line an 8×8-inch (20×20 cm) light-colored metal pan with foil or parchment and grease it.
Melt. Microwave the butter and sugar in a large microwave-safe bowl for about 1 minute, until the butter melts.
Whisk. Add the eggs and yolk and whisk vigorously for about 1 minute. Whisk in the vanilla, then stir in the oil and cocoa.
Combine. Stir in the flour, cornstarch, and salt with a spatula until just combined. Fold in the chocolate chips.
Bake. Spread the batter in the pan and bake for about 30 minutes, until set and a toothpick comes out with moist crumbs. Don’t overbake.
Cool and slice. Cool completely (about 2 hours). Chill until firm if time allows, then cut into 9 large or 16 small squares.
Melt the white chocolate. Microwave the candy melts or chocolate at 50% power, stirring every 30 seconds, until smooth.
Pipe. Transfer to a piping bag with a small round tip, or a zip-top bag with a tiny corner snipped. On each square, pipe four crossing lines, then connect them with 3–4 rows of curved lines and finish with a small center circle.
Set. Let the webs set completely before serving or storing.

White chocolate choice: Melting wafers taste best and set firm at room temperature. Candy melts set fastest. White baking chocolate tastes great but may need refrigerating. Avoid white chocolate chips.
Candy melts set quickly, so melt them in small batches and work fast, especially in a cold kitchen.
Use a light-colored metal pan. If using glass, add 5–10 minutes to the bake time.
Cocoa: Dutch-process is recommended. Natural cocoa works 1:1. Don’t use cacao powder.
Double batch: Use a 9×13-inch pan, add a few minutes to the bake time, and use ⅔ cup (112 g) of white chocolate.
Storage: Airtight at room temperature for 3 days or in the fridge for 5 days. Freeze undecorated brownies for up to 1 month.

Nutrition is an estimate and will vary with the brands used.