Preheat your oven to 180°C (160°C for fan) or gas mark 4. Grease three 20cm round cake tins with butter and line the bottoms with baking parchment.

Whisk together cocoa powder with 200ml of boiling water until smooth. Incorporate 200ml of cooled chocolate stout, stirring until fully combined.

In a stand mixer, beat 250g of unsalted butter, 200g white caster sugar, and 100g light brown soft sugar on medium speed for about 5 minutes until fluffy. Gradually add 3 large lightly beaten eggs, mixing after each addition, followed by 150g melted dark chocolate.

In another bowl, whisk together 275g of plain flour, 1 teaspoon of bicarbonate of soda, and a pinch of salt. Slowly add this dry mixture into the butter-sugar-egg mixture, alternating it with the cocoa-stout mixture until just combined.

Divide the batter among the prepared cake tins and bake for 25-30 minutes or until a skewer comes out clean. Cool in the tins for 10 minutes before transferring to a wire rack to cool completely.

For the Swiss meringue buttercream, whisk 3 large egg whites and 200g caster sugar in a heatproof bowl over simmering water until sugar dissolves. Whisk on high speed until stiff peaks form and the mixture cools. Gradually add 250g room temperature unsalted butter, mixing until glossy, then fold in 100g melted white chocolate.

Place the first cake layer on a plate, spread with buttercream, add the second layer and repeat. Place the final layer and coat the entire cake with a thin layer of buttercream. Chill for 1 hour.

Melt 100g white mini marshmallows in a bowl over simmering water until smooth. Cool slightly, then stretch into strands and drape over the cake to create cobwebs. Form spider shapes from black sugar paste and place them on top.