Heat the oven to 180C/350F/Gas 4.
Break up the chocolate, put into a heatproof bowl with the diced butter and melt gently. Remove the bowl from the heat and leave to cool until needed.
Put the egg yolks and both sugars into the bowl of a food mixer.
Beat with the whisk attachment until very thick and fluffy. Gently stir in the cooled chocolate mixture and the ground almonds.
Whip the egg whites until stiff. Using a large metal spoon, very gently fold the whites into the chocolate mixture in three batches.
Spoon into the prepared tin and spread evenly. Bake in the heated oven for 30 minutes until the centre is barely set. Remove the tin from the oven and set on a wire rack. Leave to cool for 15 minutes and then carefully remove the torte from the tin. Dust with icing sugar and serve just warm or at room temperature. Best eaten within 2 days.
Photo by Lisa Barber for Divine chocolate