Pre-heat the oven to 160°C. Spray a 23cm spring-form tin with a non stick baking spray and line with a non stick baking paper.
In the microwave (or a small saucepan, over a very gentle heat) melt the chocolate and butter, stirring together to blend well. Stir in the egg yolks, Chelsea Caster Sugar, Chelsea Icing Sugar and ground almonds.
In a separate bowl with clean beaters, whisk the egg whites until stiff peaks form. Carefully fold the egg whites into the chocolate mixture and then pour into the prepared tin.
Bake for approx 1 hour, until the cake is firm in the centre. Run a palette knife around the edge of the tin to loosen the sides. Remove the spring-form ring and turn out on a wire rack to cool completely.
Serve dusted generously with extra Chelsea Icing Sugar. This cake is delicious served with a little softly whipped cream and fruit – strawberries, raspberries etc.