Heat the oven to 180°C/350°F conventional, or 160°C/320°F fan. Line a 20 cm/8-inch square baking pan with parchment paper, leaving a little overhang to help lift out the cooled brownies.

Place the cubed butter and chopped dark chocolate in a heatproof bowl set over barely simmering water, making sure the base of the bowl does not touch the water. Alternatively, melt them in short microwave intervals, stirring between each interval, until smooth, then cool slightly.

Whisk the eggs and golden caster sugar for several minutes until the mixture becomes pale, thick, and slightly foamy. This step helps create the characteristic shiny, lightly crackled brownie top.

Fold the slightly cooled melted chocolate mixture into the egg and sugar mixture until evenly combined. Use a gentle folding motion so you keep as much air in the mixture as possible.

Sift the all-purpose flour and unsweetened cocoa powder into the bowl. Fold just until no dry streaks remain, taking care not to overmix because that can make the finished brownies less tender.

Pour the batter into the parchment-lined pan and smooth the top with a spatula. Spread the mixture into the corners so the brownies bake evenly.

Bake for approximately 25 minutes. The edges should look set while the center remains slightly soft, since the brownies will continue setting as they cool.

Cool the brownies completely in the pan before cutting. For especially clean slices, chill the cooled slab briefly before lifting it out and slicing it into about 12 squares.

Melt the white chocolate gently, then pipe spiderwebs, zigzags, ghosts, or other Halloween designs over the brownies. Press edible candy eyes into the white chocolate while it is still soft, and allow the decoration to set before serving.