1. Sift flour into a large basin and sprinkle with the flaked butter. Make a well in the centre, add the sugar, egg yolk, salt and lemon rind and quickly knead to a smooth dough working from the centre outwards. Shape into a ball, wrap in cling film and leave for 2 hours in the refrigerator.
2. Preheat oven to gas mark 5/190º C. Roll out dough thinly on a floured surface to make two 10 inch rounds. Place on greased baking trays and bake for 8-10 minutes until golden. While still warm, cut one of the rounds into 8 equal portions and cool on wire rack.
3. Dissolve the gelatin in 3 tbsp water over low heat. Put milk, sugar, salt, lemon rind and egg yolks in top of a double boiler or in a basin over hot water, and cook over low heat, stirring continuously until smooth and thickened.
4. Remove from heat and add dissolved gelatin. Leave to cool. Whip cream until thick. Blend in strained yogurt and carefully fold into the cooled mixture. Line the sides of a 10 inch spring form cake tin with grease proof paper. Place uncut shortbread in the base of the tin, spoon over the cheese filling and smooth the surface.
5. Arrange the cut shortbread on top to form a complete round and cover with cling film. Allow to set in refrigerator overnight.
6. Remove the cheesecake from the spring form tin and carefully remove the grease proof paper. Sprinkle the top with sifted icing sugar.