Preheat the oven to 180°C (160°C fan) / 350°F. Grease and line an 8-inch square tin with baking paper.

In a large mixing bowl, melt the butter - either in the microwave or gently on the hob over a pan of hot water.

Stir in the sugar while the butter is still warm. Mix until glossy and well combined.

Add the eggs and vanilla, then whisk by hand or with a hand whisk for about 30–60 seconds until the mixture lightens a little in colour.

Sift in the cocoa powder, flour and salt. Stir gently until no dry spots remain - don't overmix.

Fold in the chopped chocolate and any optional extras. The batter will be thick and glossy.

Spoon the batter into your prepared tin, spread it out evenly and smooth the top.

Bake for 25–30 minutes, or until the edges are set and a skewer inserted into the middle comes out with a few moist crumbs.

Place the marshmallows in a heatproof bowl and microwave in 10–15 second bursts until melted and gooey. Stir with a spoon or fork to help them melt evenly.

Let them cool slightly. Wait about 30 seconds - just until the marshmallow is cool enough to touch but still stretchy.

Use your fingers to pinch a bit of the melted marshmallow and stretch it out into thin strands. Drape and pull it across the top of the brownies in random criss-cross patterns to mimic spiderwebs. Keep going until you’re happy with the look!

Stick on some candy eyeballs or chocolate spiders while the marshmallow is still sticky.