<p>Heat oven to 180C. Melt chocolate, sugar and butter in a bowl over a pot of simmering water. Remove from heat, stir thoroughly to combine, mix in ground almonds, then beat in the egg yolks, one at a time.</p> <p>Beat egg whites until stiff and peaked, and stir a couple of spoonfuls into the chocolate mixture to lighten it, before gently folding in the rest.</p> <p>Turn into a buttered and floured 20cm round tin and bake for 40 to 50 minutes.</p> <p>Leave to cool before removing from tin. Dust with icing sugar to serve.</p>