Preheat oven to 150C.
Melt the chocolate in a bowl over hot water off the heat.
Beat in the butter and sugar and the yolks, beaten for a few minutes.
Stiffly beat the egg whites, beat a quarter of this mass into the chocolate base and fold in the remaining whites.
Pour two thirds into the cake tin and put the remaining mixture in the fridge covered with plastic film.
Bake the cake for 40-50 minutes.
When the cake feels firm and springy, remove from the oven.
Turn it out upside down onto a serving platter and remove the spring form tin.
Allow the chocolate cake to cool for at least one hour. The centre should fall in.
Fill this space with the cold remaining mixture, dust with cocoa or chocolate and serve with whipped cream.
For 8-10 serves you will need a 26-28 cm spring form cake tin, which should be greased and lined with greaseproof paper or ‘Bakewell’
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