Line with baking paper and grease a 25 cm (10 inch) cake tin. Put the nectarines in a bowl and pour over boiling water. Leave to stand for 1 minute, then peel off the skins. Halve the nectarines and remove the stones. Drain them well on kitchen paper and arrange, cut side down, in the prepared cake tin.
Put the chocolate and butter in a heatproof bowl and melt together over a pan of simmering water.
Beat together the egg yolks and sugar until the whisk leaves a trail when lifted. The mix should be very pale and quite stiff. Stir in the chocolate and chocolate extract.
Beat the egg whites until softly peaking. Stir a spoonful into the cake mix, then fold in the remainder. Spoon the mixture over the nectarines.
Bake the cake in a preheated oven, 180°C (350°F), Gas Mark 4, for 45 minutes or until a skewer inserted comes out clean. Serve warm or cold, dusted with cocoa powder.