Heat the oven to 180℃/160℃ Fan/350°F. Butter two 20cm / 8 inch sandwich tins and line them with baking parchment.

Cut the butter lengthways into four pieces and put into a heavy-based saucepan. Set over gentle heat. Add 250ml of just-boiled water, and whisk in the cocoa and both sugars, whisking gently until the butter has melted.

Remove from the heat, and stir in the vanilla extract. Let stand for 5 minutes.

Measure out the flour in a bowl, add the baking powder and bicarbonate of soda and fork to mix. Whisk the eggs together in a small jug.

Pour the eggs gradually into the pan, whisking all the while, until they are completely absorbed.

Whisk in the flour slowly and gently until you have a smooth batter, and pour and scrape evenly into your waiting tins.

Bake in the oven for 18–20 minutes, until the cakes begin to shrink away at the edges.

Leave the cakes to cool for 10–15 minutes on a rack.

To make the buttercream, sift the icing sugar into a bowl.

In another bowl, beat the butter and peanut butter together thoroughly until light and fluffy.

Beat in the vanilla extract and salt.

Add the sifted icing sugar a spoonful at a time until incorporated, then beat for 2 minutes.

Add the cream a tablespoonful at a time and beat for 4 minutes until you have a soft, aerated mixture.

Peel away the lining papers from the cakes and place one layer on a cake stand.

Spread about a third of the icing onto the waiting cake layer.

Top with the second cake, placing it domed-side up, and spread another third of the icing over the top.

Use a spatula to cover and smooth the buttercream around the cake.