Chocolate Charlotte

Line a 15 cm round springform tin or charlotte mould with cling film, leaving enough overhang to fold over the top later. Separate the eggs into two clean bowls. Finely chop the chocolate. Combine the cooled espresso and rum (if using) in a shallow dish wide enough to dip the biscuits.

Place the chopped chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water, ensuring the bowl does not touch the water. Stir gently until fully melted and glossy, then remove from the heat and cool for 5 minutes.

Whisk the egg yolks and caster sugar together until the mixture is pale and falls in thick ribbons, about 3 minutes. Stir in the vanilla extract, then fold the melted chocolate mixture into the yolks until fully combined.

Whip the chilled double cream with the icing sugar to soft, billowy peaks — it should hold a gentle fold but not be stiff.

Add the pinch of salt to the egg whites and whisk to firm, glossy peaks that stand straight when the whisk is lifted.

Fold the whipped cream into the chocolate mixture using a large spatula, then fold in the egg whites in two additions, cutting through the centre and sweeping around the bowl to preserve as much volume as possible.

Working quickly one at a time, dip each boudoir biscuit into the espresso mixture for no more than 2 seconds per side — the biscuit should absorb liquid but still hold its shape. Arrange enough dipped biscuits in a single snug layer to cover the base of the prepared mould, trimming to fit as needed.

Spoon half the chocolate mousse over the biscuit base and spread level with a palette knife.

Lay a second layer of dipped biscuits over the mousse, again trimming to fit neatly.

Spoon the remaining mousse on top and smooth the surface flat. Fold the cling film overhang over the top to cover.

Refrigerate for a minimum of 6 hours, or overnight, until the mousse is fully set and the biscuits have melded into a coherent, sliceable layer.

To unmould, unfold the cling film, invert the charlotte onto a serving plate, and carefully lift away the mould and cling film. Dust generously with cocoa powder or scatter dark chocolate shavings over the top before slicing and serving cold.