Heat the oven to 180°C/160°C fan/350°F/ Gas 4. Put the cocoa powder, 150g of the caster sugar and the boiling water in a jug and whisk together until the sugar has dissolved. Set aside to cool slightly.
Cream the butter and remaining caster sugar together until pale and fluffy, then add the eggs, one at a time, beating well between each addition.
Sift the flour, baking powder, bicarbonate of soda, cinnamon, nutmeg and cayenne, if using, together in another bowl. Add 1/3 of the flour mixture to the creamed butter and sugar, and beat well to combine. Repeat twice more, using 1/3 of the flour mixture each time and beating well between each addition.
Fold in the cocoa and sugar mixture until fully combined. Divide the cake batter equally between the 3 lined tins and bake for 25–30 minutes, until risen and a skewer inserted into the centre comes out clean.
Remove the cakes from the oven, leave to cool in the tins for 5 minutes, then turn out onto wire racks. Peel off the paper and leave to cool completely.
To make the filling, whisk all the ingredients together, until light and aerated (it should look like bubbles in hot chocolate). Divide the mixture into 3 equal portions, using 2 portions to sandwich the cakes together and the third to cover the top and sides.
To decorate the cake, whip the cream to firm peaks, then spoon it into a large piping bag fitted with a large star nozzle. Pipe swirls of cream over the top of the cake, then sprinkle with mini marshmallows and the grated chocolate, and dust with cinnamon.
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