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

In a medium bowl, combine cocoa powder with boiling water and chocolate stout until smooth. In a large mixing bowl, beat room temperature unsalted butter, white caster sugar, light brown sugar, and vanilla extract together until fluffy. Add the eggs one by one followed by melted dark chocolate, blending well.

In another bowl, whisk together plain flour, bicarbonate of soda, and a pinch of salt. Gradually incorporate the dry ingredients into the wet mixture alternating with the cocoa-stout mixture. Mix until just combined and pour into prepared tins.

Bake for 25-30 minutes. Insert a skewer to check for doneness; it should come out clean. Cool in tins for 10 minutes, then transfer to a wire rack to cool completely.

For the buttercream, whisk egg whites and caster sugar over simmering water until sugar dissolves (about 5 minutes). Beat with an electric mixer until stiff peaks form. Gradually add room temperature unsalted butter and melted white chocolate, mixing until smooth.

Once cakes are cool, place one layer on a serving platter, spreading Swiss meringue buttercream between layers. Repeat with the second layer. Cover the entire cake with buttercream and chill in the refrigerator for 1 hour.

Melt white mini marshmallows in a microwave for about 30 seconds. Grease hands and pull the marshmallow into thin strands to drape over the cake, creating a spider web effect.

Roll black sugar paste into a small spider shape, adding legs for realism. Place atop the webbed cake.