Grease and line a 20 cm (8 inch) square, or similar-sized, loose-based tin with nonstick baking paper. Grease the paper.
Beat together the butter, caster sugar and lemon rind in a bowl until light and fluffy. Gradually beat in the eggs, adding a little flour to prevent the mixture curdling. Add the remaining flour and the ground almonds and fold the ingredients together gently until just combined. Turn into the tin and level the surface.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 25 minutes until just firm to the touch. Transfer to a wire rack. While still warm, sprinkle the granulated sugar in a thick layer over the cake and drizzle with the lemon juice. Leave to cool completely. Cut into 25 small slices.