Ingredients

For the sponge:

• 200g salted butter, plus more for the tin
• 200g plain chocolate, chopped or chips
• 240g light muscovado sugar
• 4 medium eggs free range eggs
• 175g soured cream, room temperature
• 190g self-raising flour
• 40g cocoa powder

For the buttercream frosting:

• 175g softened unsalted butter
• 350g sifted icing sugar
• 140g plain chocolate, melted then cooled slightly
• A couple of tbsp. milk to loosen
• 2 x digestive biscuits (or similar), crumbled into fairly fine crumbs with your fingers. You don’t need to blitz these as it is nice to get little bites of the biscuit as you eat the cake and frosting.

For the Swiss meringue:

• 3 egg whites
• 150g white caster sugar
• Vanilla extract
• Pinch of salt

Tools

• A stand mixer with a paddle attachment, electric hand whisk or
• bowl and wooden spoon if doing by hand
• Cake plate or stand
• Palette knife (crank handle is best)
• Turntable (optional)
• Blow torch
• 3 x 7” round cake tins (or do in batches)

Method

To the sponge:

1. Grease, base and sideline 3 x 7" round tins.
2. Preheat the oven to 160c fan.
3. Gently melt the butter, chocolate and sugar in the microwave for 30-second blasts, stirring each time, so it doesn't catch. Alternatively, you can set a large bowl over a pan of simmering water until the chocolate chips have melted and it's a smooth liquid.
4. Remove from the heat and leave to cool a little, then add the orange zest.
5. In another bowl, beat together the eggs with the vanilla and soured cream.
6. Sift the flour and cocoa powder into another bowl and whisk to distribute. Stir the egg mixture into the cooled chocolate mixture, then add in the flour mix and fold through.
7. Spoon a third of the mixture into each tin and level off with the back of a spoon. Bake for 15-18 minutes until just cooked. The secret to a good chocolate cake is to slightly underbake it, a skewer or sharp knife inserted into the centre of the cake should come out a little pasty, not dry, but not glistening - they'll firm up in the tins as they cool.
8. Allow the cakes to cool in the tins for 10 minutes, then turn them out onto a wire rack to cool completely. Be careful, as they are fragile.

For the buttercream icing:

1. Make the frosting while the cakes are baking. Beat the butter until pale and creamy. Gradually add the icing sugar, beating well between each addition.
2. Melt the chocolate in the microwave in 30-second blasts, stirring between each time to ensure it doesn't catch. Alternatively, melt in a Bain Marie. Let it cool slightly and then mix it through the buttercream, add a little milk to loosen if needed, you want the frosting to be a soft but palatable consistency that'll hold its shape and not run.

To assemble:

1. Check the cakes are flat and level so they stack nicely – if they have a little hump from the rise, you can trim them with a bread knife.
2. Put the first layer onto a cake stand and spread a layer of frosting on top right over the cake, slightly going out over the edge, and sprinkle over with some biscuit crumbs.
3. Add the second cake and sandwich it together by pressing gently with your palm. Make sure the cakes are level and in line.
4. Add another layer of frosting, and a sprinkling of biscuit crumbs, then take the third sponge and turn it upside down so that the flat (bottom) part of the sponge becomes the top and place it on top of the other two sponge layers.
5. Now we'll cover it with a thin layer of icing to fill the gaps and stick it together. Spread the excess icing that is poking out between the layers, and smooth over the sides of the cake to fill the gaps between the layers. Add a little more frosting to the top of the cake, spread it all over the top, and go around the sides, getting a neat thin coating of icing all over the cake. This is a crumb coat, which seals in any crumbs and sets the cake with a base coat. If you'd like to see a video of how to do this, I've it on my YouTube channel.
6. Put the cake into the freezer for 10 minutes or fridge for 30 minutes to allow the icing to set a little.

To assemble:

1. Whip up the Swiss meringue. I've used my Kenwood cooking chef machine on the automatic setting and just added the egg white, sugar and salt to the bowl.
2. If you wish to do this by hand, combine the egg whites, sugar, and salt in a heatproof bowl of an electric mixer, and place over a saucepan of just simmering water. Whisk constantly until the sugar is dissolved and the whites are warm to the touch. This will take about 4-5 minutes. The sugar should feel dissolved entirely, and the mix should be silky smooth with no granules.
3. Transfer bowl to electric mixer fitted with the whisk attachment, and whip, starting on low speed, gradually increasing to high until stiff, glossy peaks form, about 8-10 minutes. Add vanilla, and mix until combined. Or do it with an electric hand whisk.
4. Pile the meringue on top of the cake and spread it all over the top and sides, using a palette knife or spoon to create lovely swirly flicks and peaks. You can leave it like this, but it looks gorgeous toasted with a blow torch. Toasting the meringue smells incredible and brings the S'mores experience to life on your cake. Serve with mini sparklers if you fancy a bit of drama!
5. Best served on the day when freshly toasted, but the cake will keep for up to 5 days in the fridge, but bring to room temperature before serving.