Place the almonds in large, dry frying pan over a low heat and toast for 5–10 minutes, shaking the pan frequently, until lightly browned. Leave to cool, then tip into a food processor and pulse until finely ground.
Beat the egg yolks in a bowl until pale and thickened, then beat in the sugar, citrus rind and cinnamon. Stir in the ground almonds.
Whisk the egg whites in a large, clean bowl until stiff peaks form. Stir one-third into the almond mixture to loosen, then gently fold in the remaining mixture in 2 batches. Spoon the mixture into a greased and base-lined 23 cm (9 inch) springform cake tin.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 40 minutes or until the sides start to shrink away from the tin. Leave to cool in the tin for 10 minutes, then turn out on to a wire rack and leave to cool. Dust with icing sugar, cut into slices and serve with crème fraîche.