Butter a 15 cm/6 in cake tin (or a small loaf tin), put a disc of buttered baking parchment on the bottom and dust with flour. Extract the seeds from the cardamom pods and break up the cinnamon roughly. Put the spices into a coffee grinder and whizz until finely ground. Put a bowl over a pan of simmering water, making sure the water does not touch the base of the bowl. Put the ground spices into the bowl and leave them for 2-3 minutes, stirring once or twice to release their aroma. Break up the chocolate and add it with the water to the bowl. Turn the heat right down, or off, and stir constantly until the chocolate is melted and smooth. Remove from the heat.
Preheat the oven to 180°C/350°F/gas mark 4. Separate the eggs into 2 bowls, a larger one for the whites. Whisk half the sugar with the egg yolks until pale and very thick, then stir in the melted chocolate, salt, cocoa powder, yoghurt and breadcrumbs. Whisk the egg whites until stiff and then gradually fold in the remaining sugar before folding into the chocolate mixture. The mixture should almost be a dropping consistency. If it is very stiff, stir in a tablespoon of water.
Spoon the mixture into the cake tin and cook in the centre of the oven for 30-40 minutes, until it feels firm to a light touch in the centre. Remove from the oven and leave in the tin for 5-10 minutes. Then loosen the edges with a knife and turn out, right-side up, on to a rack to cool. Remove the disc of baking parchment. When cold, wrap in cling film and keep in a cool place until needed. Before serving, lightly sprinkle icing sugar through a sieve on top of the cake.