Chocolate orange cake

15og dark chocolate
100g butter
6 large eggs
150g ground almonds
finely grated zest of 2 med oranges
2tbsp freshly squeezed orange juice
100g caster sugar
3tbsp cooca

Preheat oven to 170c/150c fan/gas 3. Grease a round 23cm-deep cake tin with butter and line the base with baking paper.Break the chocolate into squares and put in a bowl with the butter.Stand the bowl over a pan of simmering water until chocolate has melted-make sure the base  doesn't touch the water.Leave to cool slightly.Separate egg yolks from whites.Stir the ground almonds,orange zest,juice and yolks into the chocolate.In a different bowl,whisk the egg whites until stiff.Add the sugar 1tbsp at a time-after each addition,whisk until stiff again.Add a quarter to the chocolate mixture and gently fold in.Then fold in the rewst of the egg whites.Turn the mixture into the cake tin and bake for 35-40min.Leave to cool in the tin for 10min,then turn out and finish cooling on a wire rack.Serve dusted with the cocoa.