150g soft unsalted butter, plus some for greasing
250g caster sugar
100g ground almonds
4 teaspoons best-quality cocoa powder, sifted
1 x 23cm springform or other round cake tin
Preheat the oven to 180°C/gas mark 4, line the base of your cake tin with baking parchment and butter the sides.
Beat the eggs and sugar together until about tripled in volume, pale and moussy. I use a freestanding mixer, but a hand-held electric model would be just fine too.
Mix the ground almonds with the cocoa powder and fold this gently into the egg and sugar mixture, followed by the slightly cooled chocolate and butter. Finally, fold in the zest and juice of your lime. Folding gently will keep the air in and give the cake it's lightness.
Pour and scrape this mixture into the prepared tin and bake in the preheated oven for 40–45 minutes (though start to check at 35); the cake will be just firm on top, but still have a bit of wobble underneath.
Remove from the oven and sit the cake in its tin on a wire rack to cool. Once the first heat has left it, drape a clean tea towel over the cake to stop it getting too crusty, though a cracked and cratered surface is to be expected.
When cold, unmould, dust with icing sugar.