Melt the chocolate, butter and cream together in a heatproof bowl set over a saucepan of gently simmering water. Remove from the heat and leave to cool for 5 minutes.
Whisk the egg yolks with 75 g (3 oz) of the sugar until pale and stir in the cooled chocolate mixture.
Whisk the egg whites in a large clean bowl until softly peaking then whisk in the remaining sugar. Fold into the egg yolk mixture with the sifted cocoa powder until evenly incorporated.
Pour the cake mixture into an oiled and base-lined 23 cm (9 inch) spring-form cake tin that has been lightly dusted all over with a little extra cocoa powder. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 35 minutes.
Leave to cool in the tin for 10 minutes then turn out on to a serving plate. Serve in wedges, while still warm, with whipped cream and strawberries.